Br failed due to checksum exceeding the deadline in 8c cluster

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
Just do one data import.
Plan: 842763

### 2. What did you expect to see? (Required)
Import succeeded.

### 3. What did you see instead (Required)
Br failed due to following error.
[2022/06/02 23:01:33.248 +00:00] [INFO] [coprocessor.go:852] ["[TIME_COP_PROCESS] resp_time:977.331851ms txnStartTS:433641416266350594 region_id:107284 store_addr:tc-tikv-1.tc-tikv-peer.endless-oltp-tps-842763-1-437.svc:20160 kv_process_ms:977 kv_wait_ms:0 kv_read_ms:0 processed_versions:1830971 total_versions:1830972 rocksdb_delete_skipped_count:0 rocksdb_key_skipped_count:1830971 rocksdb_cache_hit_count:1 rocksdb_read_count:1356 rocksdb_read_byte:35926514"]
[2022/06/02 23:01:33.570 +00:00] [INFO] [coprocessor.go:852] ["[TIME_COP_PROCESS] resp_time:1m0.012045762s txnStartTS:433641419412078596 region_id:109320 store_addr:tc-tikv-3.tc-tikv-peer.endless-oltp-tps-842763-1-437.svc:20160 kv_process_ms:59581 kv_wait_ms:420 kv_read_ms:0 processed_versions:1135616 total_versions:1135617 rocksdb_delete_skipped_count:0 rocksdb_key_skipped_count:1135616 rocksdb_cache_hit_count:3 rocksdb_read_count:4144 rocksdb_read_byte:112126404"]
[2022/06/02 23:01:33.570 +00:00] [WARN] [coprocessor.go:981] ["other error"] [txnStartTS=433641419412078596] [regionID=109320] [storeAddr=tc-tikv-3.tc-tikv-peer.endless-oltp-tps-842763-1-437.svc:20160] [error="other error: Coprocessor task terminated due to exceeding the deadline"] [errorVerbose="other error: Coprocessor task terminated due to exceeding the deadline\ngithub.com/pingcap/tidb/store/copr.(*copIteratorWorker).handleCopResponse\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/store/copr/coprocessor.go:980\ngithub.com/pingcap/tidb/store/copr.(*copIteratorWorker).handleTaskOnce\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/store/copr/coprocessor.go:792\ngithub.com/pingcap/tidb/store/copr.(*copIteratorWorker).handleTask\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/store/copr/coprocessor.go:676\ngithub.com/pingcap/tidb/store/copr.(*copIteratorWorker).run\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/store/copr/coprocessor.go:418\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_amd64.s:1571"]

### 4. What is your TiDB version? (Required)

./br -V
Release Version: v6.1.0
Git Commit Hash: edd2f0afc5be493ff71f0b411aa7318325c5cd80
Git Branch: heads/refs/tags/v6.1.0
Go Version: go1.18.2
UTC Build Time: 2022-06-01 07:58:28
Race Enabled: false

./tidb-server -V
Release Version: v6.1.0
Edition: Community
Git Commit Hash: 4c39b097d5b158924590b59bcdce0bf5aad657b3
Git Branch: heads/refs/tags/v6.1.0
UTC Build Time: 2022-06-02 14:00:36
GoVersion: go1.18.2
Race Enabled: false