mptcp: fix compilation error without MPTCP

Fixing these errors:

  net/socket.c: In function '__sock_create':
  net/socket.c:1416:13: error: 'sysctl_mptcp_enabled' undeclared (first use in this function)
   1416 |         if (sysctl_mptcp_enabled && old_protocol == IPPROTO_MPTCP &&
        |             ^~~~~~~~~~~~~~~~~~~~
  net/socket.c:1416:13: note: each undeclared identifier is reported only once for each function it appears in
  net/socket.c:1418:17: error: implicit declaration of function 'mptcp_enable_sock'; did you mean 'mptcp_del_sock'? [-Werror=implicit-function-declaration]
   1418 |                 mptcp_enable_sock(sock->sk);
        |                 ^~~~~~~~~~~~~~~~~
        |                 mptcp_del_sock
  cc1: some warnings being treated as errors
  make[2]: *** [scripts/Makefile.build:262: net/socket.o] Error 1

sysctl_mptcp_enabled is an external variable, only defined if
CONFIG_MPTCP is set. We could create new functions doing nothing but the
two undeclared identifiers are only used here. It looks then OK to add
a new 'ifdef' here, clearly showing nothing is done when MPTCP is not
enabled.

Fixes: 8785d11 ("mptcp: Allow using IPPROTO_MPTCP")
Reviewed-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

net/socket.c

@@ -1424,9 +1424,11 @@ int __sock_create(struct net *net, int family, int type, int protocol,
 	if (err < 0)
 		goto out_module_put;
 
+#ifdef CONFIG_MPTCP
 	if (sysctl_mptcp_enabled && old_protocol == IPPROTO_MPTCP &&
 	    type == SOCK_STREAM && (family == AF_INET || family == AF_INET6))
 		mptcp_enable_sock(sock->sk);
+#endif
 
 	/*
 	 * Now to bump the refcnt of the [loadable] module that owns this