NLM: Defend against file_lock changes after vfs_test_lock()

Instead of trusting that struct file_lock returns completely unchanged
after vfs_test_lock() when there's no conflicting lock, stash away our
nlm_lockowner reference so we can properly release it for all cases.

This defends against another file_lock implementation overwriting fl_owner
when the return type is F_UNLCK.

Reported-by: Roberto Bergantinos Corpas <rbergant@redhat.com>
Tested-by: Roberto Bergantinos Corpas <rbergant@redhat.com>
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>