mptcp: accurate SIOCOUTQ for fallback socket

The MPTCP SIOCOUTQ implementation is not very accurate in
case of fallback: it only measures the data in the MPTCP-level
write queue, but it does not take in account the subflow
write queue utilization. In case of fallback the first can be
empty, while the latter is not.

The above produces sporadic self-tests issues and can foul
legit user-space application.

Fix the issue additionally querying the subflow in case of fallback.

Fixes: 644807e ("mptcp: add SIOCINQ, OUTQ and OUTQNSD ioctls")
Closes: #260
Reported-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/protocol.c

@@ -3294,6 +3294,17 @@ static int mptcp_ioctl_outq(const struct mptcp_sock *msk, u64 v)
  return 0;
 
  delta = msk->write_seq - v;
+ if (__mptcp_check_fallback(msk) && msk->first) {
+ struct tcp_sock *tp = tcp_sk(msk->first);
+
+ /* the first subflow is disconnected after close - see
+ * __mptcp_close_ssk(). tcp_disconnect() moves the write_seq
+ * so ignore that status, too.
+ */
+ if (!((1 << msk->first->sk_state) &
+ (TCPF_SYN_SENT | TCPF_SYN_RECV | TCPF_CLOSE)))
+ delta += READ_ONCE(tp->write_seq) - tp->snd_una;
+ }
  if (delta > INT_MAX)
  delta = INT_MAX;