lockd: Remove unused parameter to nlmsvc_testlock()

The nlm_cookie parameter has been unused since commit 09802fd2
("lockd: rip out deferred lock handling from testlock codepath").

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>