This patch extracts xfrm_lookup() from ip6_dst_lookup()
to support source routing appropriately.

This is because xfrm_lookup() should be performed with the final
destination while ip6_dst_lookup() is called with the next-hop.
Signed-off-by: Kazunori Miyazawa <kazunori@miyazawa.org>
Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>