cifs: Fix missing fscache invalidation

A network filesystem needs to implement a netfslib hook to invalidate
fscache if it's to be able to use the cache.

Fix cifs to implement the cache invalidation hook.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Cc: stable@vger.kernel.org
Fixes: 3ee1a1fc

 ("cifs: Cut over to using netfslib")
Signed-off-by: Steve French <stfrench@microsoft.com>