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>

include/net/ipv6.h

@@ -512,12 +512,17 @@ extern void			ip6_flush_pending_frames(struct sock *sk);
 extern int			ip6_dst_lookup(struct sock *sk,
 					       struct dst_entry **dst,
 					       struct flowi *fl);
+extern struct dst_entry *	ip6_dst_lookup_flow(struct sock *sk,
+						    struct flowi *fl,
+						    const struct in6_addr *final_dst,
+						    bool want_blackhole);
+extern struct dst_entry *	ip6_sk_dst_lookup_flow(struct sock *sk,
+						       struct flowi *fl,
+						       const struct in6_addr *final_dst,
+						       bool want_blackhole);
 extern int			ip6_dst_blackhole(struct sock *sk,
 						  struct dst_entry **dst,
 						  struct flowi *fl);
-extern int			ip6_sk_dst_lookup(struct sock *sk,
-						  struct dst_entry **dst,
-						  struct flowi *fl);
 
 /*
  *	skb processing functions