[TCP]: Do not use inet->id of global tcp_socket when sending RST.

The problem is in ip_push_pending_frames(), which uses:

        if (!df) {
                __ip_select_ident(iph, &rt->u.dst, 0);
        } else {
                iph->id = htons(inet->id++);
        }

instead of ip_select_ident().

Right now I think the code is a nonsense. Most likely, I copied it from
old ip_build_xmit(), where it was really special, we had to decide
whether to generate unique ID when generating the first (well, the last)
fragment.

In ip_push_pending_frames() it does not make sense, it should use plain
ip_select_ident() instead.
Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -1249,11 +1249,7 @@ int ip_push_pending_frames(struct sock *sk)
 	iph->tos = inet->tos;
 	iph->tot_len = htons(skb->len);
 	iph->frag_off = df;
-	if (!df) {
-		__ip_select_ident(iph, &rt->u.dst, 0);
-	} else {
-		iph->id = htons(inet->id++);
-	}
+	ip_select_ident(iph, &rt->u.dst, sk);
 	iph->ttl = ttl;
 	iph->protocol = sk->sk_protocol;
 	iph->saddr = rt->rt_src;