nfsd4: fix lockowner matching

Lockowners are looked up by file as well as by owner, but we were
forgetting to do a comparison on the file.  This could cause an
incorrect result from lockt.

(Note looking up the inode from the lockowner is pretty awkward here.
The data structures need fixing.)

Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>