lockd: Fix invalid lockowner cast after vfs_test_lock

[ Upstream commit cd2d644ddba183ec7b451b7c20d5c7cc06fcf0d7 ]

After calling vfs_test_lock() the pointer to a conflicting lock can be
returned, and that lock is not guarunteed to be owned by nlm.  In that
case, we cannot cast it to struct nlm_lockowner.  Instead return the pid
of that conflicting lock.

Fixes: 646d73e9 ("lockd: Show pid of lockd for remote locks")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>