nfsd: fix unused "nn" variable warning in free_client()

If CONFIG_LOCKDEP is disabled, then there would be a warning like this:

  CC [M]  fs/nfsd/nfs4state.o
fs/nfsd/nfs4state.c: In function ‘free_client’:
fs/nfsd/nfs4state.c:1051:19: warning: unused variable ‘nn’ [-Wunused-variable]

So, let's add "maybe_unused" tag to this variable.
Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1048,7 +1048,7 @@ static struct nfs4_client *alloc_client(struct xdr_netobj name)
 static inline void
 free_client(struct nfs4_client *clp)
 {
-	struct nfsd_net *nn = net_generic(clp->net, nfsd_net_id);
+	struct nfsd_net __maybe_unused *nn = net_generic(clp->net, nfsd_net_id);
 
 	lockdep_assert_held(&nn->client_lock);
 	while (!list_empty(&clp->cl_sessions)) {