tcp: use REXMIT_NEW instead of magic number

REXMIT_NEW is a macro for "FRTO-style
transmit of unsent/new packets", this patch
makes it more readable.
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -3550,7 +3550,7 @@ static void tcp_xmit_recovery(struct sock *sk, int rexmit)
 	if (rexmit == REXMIT_NONE || sk->sk_state == TCP_SYN_SENT)
 		return;
 
-	if (unlikely(rexmit == 2)) {
+	if (unlikely(rexmit == REXMIT_NEW)) {
 		__tcp_push_pending_frames(sk, tcp_current_mss(sk),
 					  TCP_NAGLE_OFF);
 		if (after(tp->snd_nxt, tp->high_seq))