mptcp: cleanup error path in subflow_check_data_avail()

Move fallback code out of the main loop and adds some
branch prediction annotations.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/subflow.c

@@ -1109,21 +1109,12 @@ static bool subflow_check_data_avail(struct sock *ssk)
 
 		status = get_mapping_status(ssk, msk);
 		trace_subflow_check_data_avail(status, skb_peek(&ssk->sk_receive_queue));
-		if (status == MAPPING_INVALID) {
+		if (unlikely(status == MAPPING_INVALID)) {
 			ssk->sk_err = EBADMSG;
 			goto fatal;
 		}
-		if (status == MAPPING_DUMMY) {
-			__mptcp_do_fallback(msk);
-			skb = skb_peek(&ssk->sk_receive_queue);
-			subflow->map_valid = 1;
-			subflow->map_seq = READ_ONCE(msk->ack_seq);
-			subflow->map_data_len = skb->len;
-			subflow->map_subflow_seq = tcp_sk(ssk)->copied_seq -
-						   subflow->ssn_offset;
-			subflow->data_avail = MPTCP_SUBFLOW_DATA_AVAIL;
-			return true;
-		}
+		if (unlikely(status == MAPPING_DUMMY))
+			goto fallback;
 
 		if (status != MAPPING_OK)
 			goto no_data;
@@ -1167,6 +1158,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 no_data:
 	subflow_sched_work_if_closed(msk, ssk);
 	return false;
+
 fatal:
 	/* fatal protocol error, close the socket */
 	/* This barrier is coupled with smp_rmb() in tcp_poll() */
@@ -1178,6 +1170,16 @@ static bool subflow_check_data_avail(struct sock *ssk)
 	tcp_send_active_reset(ssk, GFP_ATOMIC);
 	subflow->data_avail = 0;
 	return false;
+
+fallback:
+	__mptcp_do_fallback(msk);
+	skb = skb_peek(&ssk->sk_receive_queue);
+	subflow->map_valid = 1;
+	subflow->map_seq = READ_ONCE(msk->ack_seq);
+	subflow->map_data_len = skb->len;
+	subflow->map_subflow_seq = tcp_sk(ssk)->copied_seq - subflow->ssn_offset;
+	subflow->data_avail = MPTCP_SUBFLOW_DATA_AVAIL;
+	return true;
 }
 
 bool mptcp_subflow_data_available(struct sock *sk)