flow: Extend flow informations with xfrm interface id.

Add a new flowi_xfrm structure with informations needed to do
a xfrm lookup. At the moment it keeps the informations about
the new xfrm interface id needed to lookup xfrm interfaces
that are introduced with a followup patch. We need this new
lookup key as other possible keys, like the ifindex is
already part of the xfrm selector and used as a key to
enforce the output device after the transformation in the
policy/state lookup.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Shannon Nelson <shannon.nelson@oracle.com>
Acked-by: Benedict Wong <benedictwong@google.com>
Tested-by: Benedict Wong <benedictwong@google.com>
Tested-by: Antony Antony <antony@phenome.org>
Reviewed-by: Eyal Birger <eyal.birger@gmail.com>