mptcp: add the fallback check

This patch adds the fallback check in subflow_check_data_avail(). Only
do the fallback when the msk hasn't fallen back yet.
Suggested-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NGeliang Tang <geliang.tang@suse.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/mptcp/subflow.c

@@ -1203,6 +1203,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 	return false;
 
 fallback:
+	if (!__mptcp_check_fallback(msk)) {
 		/* RFC 8684 section 3.7. */
 		if (subflow->send_mp_fail) {
 			if (mptcp_has_another_subflow(ssk)) {
@@ -1232,6 +1233,8 @@ static bool subflow_check_data_avail(struct sock *ssk)
 		}
 
 		__mptcp_do_fallback(msk);
+	}
+
 	skb = skb_peek(&ssk->sk_receive_queue);
 	subflow->map_valid = 1;
 	subflow->map_seq = READ_ONCE(msk->ack_seq);