tcp: fix signed/unsigned comparison

Kernel test robot reported:

smatch warnings:
net/ipv4/tcp_input.c:5966 tcp_rcv_established() warn: unsigned 'reason' is never less than zero.

I actually had one packetdrill failing because of this bug,
and was about to send the fix :)

v2: Andreas Schwab also pointed out that @reason needs to be negated
    before we reach tcp_drop_reason()

Fixes: 4b506af9 ("tcp: add two drop reasons for tcp_ack()")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: Nkernel test robot <lkp@intel.com>
Reported-by: NAndreas Schwab <schwab@linux-m68k.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -5959,9 +5959,10 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
 
 step5:
 	reason = tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT);
-	if (reason < 0)
+	if ((int)reason < 0) {
+		reason = -reason;
 		goto discard;
-
+	}
 	tcp_rcv_rtt_measure_ts(sk, skb);
 
 	/* Process urgent data. */