mptcp: more accurate NL event generation

Currently the mptcp code generate a "new listener" event even
if the actual listen() syscall fails. Address the issue moving
the event generation call under the successful branch.

Cc: stable@vger.kernel.org
Fixes: f8c9dfb ("mptcp: add pm listener events")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230725-send-net-20230725-v1-2-6f60fe7137a9@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -3723,10 +3723,9 @@ static int mptcp_listen(struct socket *sock, int backlog)
 	if (!err) {
 		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
 		mptcp_copy_inaddrs(sk, ssock->sk);
+		mptcp_event_pm_listener(ssock->sk, MPTCP_EVENT_LISTENER_CREATED);
 	}
 
-	mptcp_event_pm_listener(ssock->sk, MPTCP_EVENT_LISTENER_CREATED);
-
 unlock:
 	release_sock(sk);
 	return err;