mptcp: support IPV6_V6ONLY setsockopt

Without this, Opensshd fails to open an ipv6 socket listening
socket:
  error: setsockopt IPV6_V6ONLY: Operation not supported
  error: Bind to port 22 on :: failed: Address already in use.

Opensshd opens an ipv4 and and ipv6 listening socket, but because
IPV6_V6ONLY setsockopt fails, the port number is already in use.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -1632,6 +1632,33 @@ static int mptcp_setsockopt_sol_socket(struct mptcp_sock *msk, int optname,
 	return sock_setsockopt(sk->sk_socket, SOL_SOCKET, optname, optval, optlen);
 }
 
+static int mptcp_setsockopt_v6(struct mptcp_sock *msk, int optname,
+			       char __user *optval, unsigned int optlen)
+{
+	struct sock *sk = (struct sock *)msk;
+	int ret = -EOPNOTSUPP;
+	struct socket *ssock;
+
+	switch (optname) {
+	case IPV6_V6ONLY:
+		lock_sock(sk);
+		ssock = __mptcp_nmpc_socket(msk);
+		if (!ssock) {
+			release_sock(sk);
+			return -EINVAL;
+		}
+
+		ret = tcp_setsockopt(ssock->sk, SOL_IPV6, optname, optval, optlen);
+		if (ret == 0)
+			sk->sk_ipv6only = ssock->sk->sk_ipv6only;
+
+		release_sock(sk);
+		break;
+	}
+
+	return ret;
+}
+
 static int mptcp_setsockopt(struct sock *sk, int level, int optname,
 			    char __user *optval, unsigned int optlen)
 {
@@ -1655,6 +1682,9 @@ static int mptcp_setsockopt(struct sock *sk, int level, int optname,
 	if (ssk)
 		return tcp_setsockopt(ssk, level, optname, optval, optlen);
 
+	if (level == SOL_IPV6)
+		return mptcp_setsockopt_v6(msk, optname, optval, optlen);
+
 	return -EOPNOTSUPP;
 }