xfrm: policy: split list insertion into a helper

... so we can reuse this later without code duplication when we add
policy to a second inexact list.

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>