extended.openjdk failure: java/nio/channels/SocketChannel/AdaptSocket.java : Incorrect number of bytes read: 15

https://ci.adoptopenjdk.net/job/Test_openjdk8_j9_extended.openjdk_x86-64_linux/25/consoleFull
Hotspot and OpenJ9:
```
12:30:22 openjdk version "1.8.0_292"
12:30:22 OpenJDK Runtime Environment (build 1.8.0_292-202103091156-b05)
12:30:22 Eclipse OpenJ9 VM (build openj9-0.26.0-m1, JRE 1.8.0 Linux amd64-64-Bit Compressed References 20210309_972 (JIT enabled, AOT enabled)
12:30:22 OpenJ9 - b227feba2
12:30:22 OMR - 4665e2f72
12:30:22 JCL - 1780cbc92b based on jdk8u292-b05)
```
```
12:57:20 opened: Socket[unconnected]
12:57:20 java.nio.channels.SocketChannel[unconnected]
12:57:20 connected: Socket[addr=f097c0fac40a/172.17.0.4,port=41865,localport=58538]
12:57:20 java.nio.channels.SocketChannel[connected local=localhost/127.0.0.1:58538 remote=f097c0fac40a/172.17.0.4:41865]
12:57:20 f097c0fac40a/172.17.0.4:41865 says: "Tue Mar 09 12:57:19 UTC 20"
12:57:20 ishut: java.nio.channels.SocketChannel[connected ishut local=localhost/127.0.0.1:58538 remote=f097c0fac40a/172.17.0.4:41865]
12:57:20 oshut: java.nio.channels.SocketChannel[connected ishut oshut local=localhost/127.0.0.1:58538 remote=f097c0fac40a/172.17.0.4:41865]
12:57:20 closed: Socket[unconnected]
12:57:20 java.nio.channels.SocketChannel[closed]
12:57:20
12:57:20 opened: Socket[unconnected]
12:57:20 java.nio.channels.SocketChannel[unconnected]
12:57:20 connected: Socket[addr=f097c0fac40a/172.17.0.4,port=41865,localport=58546]
12:57:20 java.nio.channels.SocketChannel[connected local=localhost/127.0.0.1:58546 remote=f097c0fac40a/172.17.0.4:41865]
12:57:20 f097c0fac40a/172.17.0.4:41865 says: "Tue Mar 09 12:57:19 UTC 20"
12:57:20 ishut: java.nio.channels.SocketChannel[connected ishut local=localhost/127.0.0.1:58546 remote=f097c0fac40a/172.17.0.4:41865]
12:57:20 oshut: java.nio.channels.SocketChannel[connected ishut oshut local=localhost/127.0.0.1:58546 remote=f097c0fac40a/172.17.0.4:41865]
12:57:20 closed: Socket[unconnected]
12:57:20 java.nio.channels.SocketChannel[closed]
12:57:20
12:57:20 opened: Socket[unconnected]
12:57:20 java.nio.channels.SocketChannel[unconnected]
12:57:20 connected: Socket[addr=f097c0fac40a/172.17.0.4,port=45183,localport=56156]
12:57:20 java.nio.channels.SocketChannel[connected local=localhost/127.0.0.1:56156 remote=f097c0fac40a/172.17.0.4:45183]
12:57:20 f097c0fac40a/172.17.0.4:45183 says: "Tue Mar 09 12:57:19 UTC 20"
12:57:20 ishut: java.nio.channels.SocketChannel[connected ishut local=localhost/127.0.0.1:56156 remote=f097c0fac40a/172.17.0.4:45183]
12:57:20 oshut: java.nio.channels.SocketChannel[connected ishut oshut local=localhost/127.0.0.1:56156 remote=f097c0fac40a/172.17.0.4:45183]
12:57:20 closed: Socket[unconnected]
12:57:20 java.nio.channels.SocketChannel[closed]
12:57:20
12:57:20 connected: Socket[addr=f097c0fac40a/172.17.0.4,port=38291,localport=58244]
12:57:20 java.nio.channels.SocketChannel[connected local=localhost/127.0.0.1:58244 remote=f097c0fac40a/172.17.0.4:38291]
12:57:20 timeout: 0
12:57:20
12:57:20 connected: Socket[addr=f097c0fac40a/172.17.0.4,port=38291,localport=58250]
12:57:20 java.nio.channels.SocketChannel[connected local=localhost/127.0.0.1:58250 remote=f097c0fac40a/172.17.0.4:38291]
12:57:20 timeout: 8000
12:57:20
12:57:20 connected: Socket[addr=f097c0fac40a/172.17.0.4,port=33377,localport=45088]
12:57:20 java.nio.channels.SocketChannel[connected local=localhost/127.0.0.1:45088 remote=f097c0fac40a/172.17.0.4:33377]
12:57:20 timeout: 10
12:57:20 Read timed out, as expected
12:57:20 Read timed out, as expected
12:57:20 Read timed out, as expected
12:57:20 Read timed out, as expected
12:57:20 STDERR:
12:57:20 java.lang.Exception: Incorrect number of bytes read: 15
12:57:20 at AdaptSocket.testRead(AdaptSocket.java:104)
12:57:20 at AdaptSocket.testRead(AdaptSocket.java:138)
12:57:20 at AdaptSocket.main(AdaptSocket.java:168)
12:57:20 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
12:57:20 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
12:57:20 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
12:57:20 at java.lang.reflect.Method.invoke(Method.java:498)
12:57:20 at com.sun.javatest.regtest.agent.MainActionHelper$AgentVMRunnable.run(MainActionHelper.java:312)
12:57:20 at java.lang.Thread.run(Thread.java:823)
12:57:20
12:57:20 JavaTest Message: Test threw exception: java.lang.Exception
```

Start with java/nio/channels/SocketChannel/AdaptSocket.java, especially testRead at line 104, and review the linked CI console output. Reproduce the extended.openjdk failure and trace why the test reports 15 bytes read. Done means the test's expected byte count is correct and the CI test passes without the exception.

Written by the indexing model from the issue text.