xfrm interface: store xfrmi contexts in a hash by if_id

xfrmi_lookup() is called on every packet. Using a single list for
looking up if_id becomes a bottleneck when having many xfrm interfaces.

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>