NFSD: Modernize nfsd4_release_lockowner()

[ Upstream commit bd8fdb6e ]

Refactor: Use existing helpers that other lock operations use. This
change removes several automatic variables, so re-organize the
variable declarations for readability.

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