NFSD: Clean up unused variable

@sb should have been removed by commit 7e64c5bc ("NLM/NFSD: Fix
lock notifications for async-capable filesystems").

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>