nfsd: remove unneeded spinlock in nfsd_cache_update

The locking rules for cache entries say that locking the cache_lock
isn't needed if you're just touching the current entry. Earlier
in this function we set rp->c_state to RC_UNUSED without any locking,
so I believe it's ok to do the same here.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>