nlm: minor refactoring

[ Upstream commit a81041b7 ]

Make this lookup slightly more concise, and prepare for changing how we
look this up in a following patch.

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>