ipv4: fix ip header ident selection in __ip_make_skb()

Christian Casteyde reported a kmemcheck 32-bit read from uninitialized
memory in __ip_select_ident().

It turns out that __ip_make_skb() called ip_select_ident() before
properly initializing iph->daddr.

This is a bug uncovered by commit 1d861aa4b3fb (inet: Minimize use of
cached route inetpeer.)

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=46131

Reported-by: Christian Casteyde <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ip_output.c

···
 	iph->ihl = 5;
 	iph->tos = inet->tos;
 	iph->frag_off = df;
-	ip_select_ident(iph, &rt->dst, sk);
 	iph->ttl = ttl;
 	iph->protocol = sk->sk_protocol;
 	ip_copy_addrs(iph, fl4);
+	ip_select_ident(iph, &rt->dst, sk);
 
 	if (opt) {
 		iph->ihl += opt->optlen>>2;