nlm: minor nlm_lookup_file argument change

[ Upstream commit 2dc6f19e ]

It'll come in handy to get the whole nlm_lock.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>