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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/mptcp/subflow.c

···
 	 */
 	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);