mptcp: don't return sockets in foreign netns

mptcp_token_get_sock() may return a mptcp socket that is in
a different net namespace than the socket that received the token value.

The mptcp syncookie code path had an explicit check for this,
this moves the test into mptcp_token_get_sock() function.

Eventually token.c should be converted to pernet storage, but
such change is not suitable for net tree.

Fixes: 2c5ebd001d4f0 ("mptcp: refactor token container")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/mptcp/mptcp_diag.c b/net/mptcp/mptcp_diag.c
index f48eb63..292374f 100644
--- a/net/mptcp/mptcp_diag.c
+++ b/net/mptcp/mptcp_diag.c
@@ -36,7 +36,7 @@ static int mptcp_diag_dump_one(struct netlink_callback *cb,
 	struct sock *sk;
 
 	net = sock_net(in_skb->sk);
-	msk = mptcp_token_get_sock(req->id.idiag_cookie[0]);
+	msk = mptcp_token_get_sock(net, req->id.idiag_cookie[0]);
 	if (!msk)
 		goto out_nosk;