copy_file_range: fix fallback when source create on same txg

In 019dea0a5 we removed the conversion from EAGAIN->EXDEV inside
zfs_clone_range(), but forgot to add a test for EAGAIN to the
copy_file_range() entry points to trigger fallback to a content copy.

This commit fixes that.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15170
Closes #15172