mptcp: always parse mptcp options for MPC reqsk

In subflow_syn_recv_sock() we currently skip options parsing
for OoO packet, given that such packets may not carry the relevant
MPC option.

If the peer generates an MPC+data TSO packet and some of the early
segments are lost or get reorder, we server will ignore the peer key,
causing transient, unexpected fallback to TCP.

The solution is always parsing the incoming MPTCP options, and
do the fallback only for in-order packets. This actually cleans
the existing code a bit.

Reported-by:  Matthieu Baerts <matthieu.baerts@tessares.net>
Fixes: d22f498 ("mptcp: process MP_CAPABLE data option")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/subflow.c

@@ -633,21 +633,20 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 
  /* if the sk is MP_CAPABLE, we try to fetch the client key */
  if (subflow_req->mp_capable) {
- if (TCP_SKB_CB(skb)->seq != subflow_req->ssn_offset + 1) {
-  /* here we can receive and accept an in-window,
-   * out-of-order pkt, which will not carry the MP_CAPABLE
-   * opt even on mptcp enabled paths
-  */
-  goto create_msk;
- }
-
+ /* we can receive and accept an in-window, out-of-order pkt,
+  * which may not carry the MP_CAPABLE opt even on mptcp enabled
+ * paths: always try to extract the peer key, and fallback
+ * for packets missing it.
+  * Even OoO DSS packets coming legitly after dropped or
+  * reordered MPC will cause fallback, but we don't have other
+  * options.
+ */
  mptcp_get_options(sk, skb, &mp_opt);
  if (!mp_opt.mp_capable) {
  fallback = true;
  goto create_child;
  }
 
-create_msk:
  new_msk = mptcp_sk_clone(listener->conn, &mp_opt, req);
  if (!new_msk)
  fallback = true;