mptcp: propagate sk_ipv6only to subflows

Usually, attributes are propagated to subflows as well.

Here, if subflows are created by other ways than the MPTCP path-manager,
it is important to make sure they are in v6 if it is asked by the
userspace.
Reviewed-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>

net/mptcp/sockopt.c

@@ -1255,6 +1255,7 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 	ssk->sk_priority = sk->sk_priority;
 	ssk->sk_bound_dev_if = sk->sk_bound_dev_if;
 	ssk->sk_incoming_cpu = sk->sk_incoming_cpu;
+	ssk->sk_ipv6only = sk->sk_ipv6only;
 	__ip_sock_set_tos(ssk, inet_sk(sk)->tos);
 
 	if (sk->sk_userlocks & tx_rx_locks) {