xfrm: provide correct dst in xfrm_neigh_lookup

Fix xfrm_neigh_lookup to provide dst->path to the
neigh_lookup dst_ops method.

When skb is provided, the IP address in packet should already
match the dst->path address family. But for the non-skb case,
we should consider the last tunnel address as nexthop address.

Fixes: f894cbf8 ("net: Add optional SKB arg to dst_ops->neigh_lookup().")
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>