cp's copy_file_range vs read-only files

Running the following straightforward sequence on a `fuse-overlayfs` mount
```
echo foo > test
chmod -w test
cp test test2
```
breaks thusly
```
cp: error copying 'test' to 'test2': Permission denied
```
and leaves behind a zero-byte file:
```
-r--r--r-- 1 daemon daemon 4 Aug 1 12:48 test
-r--r--r-- 1 daemon daemon 0 Aug 1 12:48 test2
```

`strace`-ing that failing `cp` shows that `copy_file_range` is failing:
```
openat(AT_FDCWD, "test2", O_WRONLY|O_CREAT|O_EXCL, 0444) = 4
ioctl(4, BTRFS_IOC_CLONE or FICLONE, 3) = -1 EOPNOTSUPP (Operation not supported)
newfstatat(4, "", {st_mode=S_IFREG|0444, st_size=0, ...}, AT_EMPTY_PATH) = 0
fadvise64(3, 0, 0, POSIX_FADV_SEQUENTIAL) = 0
copy_file_range(3, NULL, 4, NULL, 9223372035781033984, 0) = -1 EACCES (Permission denied)
```

`strace`-ing `fuse-overlayfs` itself shows that it attempts to re-open the file rather than using the open handle it already has, resulting in different access semantics:
```
openat2(4, "./test2", {flags=O_RDWR|O_CREAT|O_EXCL|O_NOFOLLOW, mode=0444, resolve=RESOLVE_IN_ROOT}, 24) = 10
[statx and fuse traffic elided]
openat2(4, "test", {flags=O_RDONLY|O_NONBLOCK|O_NOFOLLOW, resolve=RESOLVE_IN_ROOT}, 24) = 11
openat2(4, "test2", {flags=O_WRONLY|O_NONBLOCK|O_NOFOLLOW, resolve=RESOLVE_IN_ROOT}, 24) = -1 EACCES (Permission denied)
writev(6, [{iov_base="\20\0\0\0\363\377\377\377\302\0\0\0\0\0\0\0", iov_len=16}], 1) = 16
close(11) = 0
read(6, "@\0\0\0\22\0\0\0\304\0\0\0\0\0\0\0000Q\217?\1\0\0\0\0\0\0\0\0\0\0\0"..., 16781312) = 64
close(10) = 0
writev(6, [{iov_base="\20\0\0\0\0\0\0\0\304\0\0\0\0\0\0\0", iov_len=16}], 1) = 16
```

The system running this is a pretty bog-standard Debian box, save that it's ppc64le, but that shouldn't matter here. `fuse-overlayfs` is their build of version 1.10.

Reproduce the `echo`, `chmod`, and `cp` sequence on a fuse-overlayfs mount, then compare the `cp` and fuse-overlayfs `strace` output shown here. Start by tracing the `copy_file_range` and file re-open handling; done means copying a read-only source succeeds without leaving a zero-byte destination.

Written by the indexing model from the issue text.