Patch for 3.2.x, 3.4.x IP identifier regression

commit c3b4ccb8b03769e2867fabecc078483ee6710ccf upstream.

With commits 73f156a6 ("inetpeer: get rid of ip_id_count") and
04ca6973 ("ip: make IP identifiers less predictable"), IP
identifiers are generated from a counter chosen from an array of
counters indexed by the hash of the outgoing packet header's source
address, destination address, and protocol number.  Thus, in
__ip_make_skb(), we must now call ip_select_ident() only after setting
these fields in the IP header to prevent IP identifiers from being
generated from bogus counters.

IP id sequence before fix: 18174, 5789, 5953, 59420, 59637, ...
After fix: 5967, 6185, 6374, 6600, 6795, 6892, 7051, 7288, ...
Signed-off-by: Jeffrey Knockel <jeffk@cs.unm.edu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Eric Dumazet <edumazet@google.com>
[Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>

net/ipv4/ip_output.c

@@ -1346,10 +1346,10 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	iph->ihl = 5;
 	iph->tos = inet->tos;
 	iph->frag_off = df;
-	ip_select_ident(skb, sk);
 	iph->ttl = ttl;
 	iph->protocol = sk->sk_protocol;
 	ip_copy_addrs(iph, fl4);
+	ip_select_ident(skb, sk);
 
 	if (opt) {
 		iph->ihl += opt->optlen>>2;