ipv6: Consolidate route lookup sequences.

Route lookups follow a general pattern in the ipv6 code wherein
we first find the non-IPSEC route, potentially override the
flow destination address due to ipv6 options settings, and then
finally make an IPSEC search using either xfrm_lookup() or
__xfrm_lookup().

__xfrm_lookup() is used when we want to generate a blackhole route
if the key manager needs to resolve the IPSEC rules (in this case
-EREMOTE is returned and the original 'dst' is left unchanged).

Otherwise plain xfrm_lookup() is used and when asynchronous IPSEC
resolution is necessary, we simply fail the lookup completely.

All of these cases are encapsulated into two routines,
ip6_dst_lookup_flow and ip6_sk_dst_lookup_flow.  The latter of which
handles unconnected UDP datagram sockets.

Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/syncookies.c

@@ -243,12 +243,9 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 		fl.fl_ip_dport = inet_rsk(req)->rmt_port;
 		fl.fl_ip_sport = inet_sk(sk)->inet_sport;
 		security_req_classify_flow(req, &fl);
-		if (ip6_dst_lookup(sk, &dst, &fl))
-			goto out_free;
 
-		if (final_p)
-			ipv6_addr_copy(&fl.fl6_dst, final_p);
-		if ((xfrm_lookup(sock_net(sk), &dst, &fl, sk, 0)) < 0)
+		dst = ip6_dst_lookup_flow(sk, &fl, final_p, false);
+		if (IS_ERR(dst))
 			goto out_free;
 	}