linux/copy_file_range: properly request a fallback copy on Linux <5.3

Before Linux 5.3, the filesystem's copy_file_range handler had to signal
back to the kernel that we can't fulfill the request and it should
fallback to a content copy. This is done by returning -EOPNOTSUPP.

This commit converts the EXDEV return from zfs_clone_range to
EOPNOTSUPP, to force the kernel to fallback for all the valid reasons it
might be unable to clone. Without it the copy_file_range() syscall will
return EXDEV to userspace, breaking its semantics.

Add test for copy_file_range fallbacks.  copy_file_range should always
fallback to a content copy whenever ZFS can't service the request with
cloning.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15131

tests/runfiles/linux.run

@@ -36,6 +36,7 @@ tags = ['functional', 'atime']
 
 [tests/functional/block_cloning:Linux]
 tests = ['block_cloning_copyfilerange', 'block_cloning_copyfilerange_partial',
+    'block_cloning_copyfilerange_fallback',
     'block_cloning_ficlone', 'block_cloning_ficlonerange',
     'block_cloning_ficlonerange_partial',
     'block_cloning_disabled_copyfilerange', 'block_cloning_disabled_ficlone',