mptcp: Initialize map_seq upon subflow establishment

When the other MPTCP-peer uses 32-bit data-sequence numbers, we rely on
map_seq to indicate how to expand to a 64-bit data-sequence number in
expand_seq() when receiving data.

For new subflows, this field is not initialized, thus results in an
"invalid" mapping being discarded.

Fix this by initializing map_seq upon subflow establishment time.

Fixes: f296234 ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Reviewed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -1629,6 +1629,8 @@ bool mptcp_finish_join(struct sock *sk)
 
 	ret = mptcp_pm_allow_new_subflow(msk);
 	if (ret) {
+		subflow->map_seq = msk->ack_seq;
+
 		/* active connections are already on conn_list */
 		spin_lock_bh(&msk->join_list_lock);
 		if (!WARN_ON_ONCE(!list_empty(&subflow->node)))