mptcp: Fix build with PROC_FS disabled.

net/mptcp/subflow.c: In function ‘mptcp_subflow_create_socket’:
net/mptcp/subflow.c:624:25: error: ‘struct netns_core’ has no member named ‘sock_inuse’
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/subflow.c

@@ -621,7 +621,9 @@ int mptcp_subflow_create_socket(struct sock *sk, struct socket **new_sock)
 	 */
 	sf->sk->sk_net_refcnt = 1;
 	get_net(net);
+#ifdef CONFIG_PROC_FS
 	this_cpu_add(*net->core.sock_inuse, 1);
+#endif
 	err = tcp_set_ulp(sf->sk, "mptcp");
 	release_sock(sf->sk);