xfrm: policy: consider if_id when hashing inexact policy

This avoids searches of polices that cannot match in the first
place due to different interface id by placing them in different bins.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>