ipcomp: assign if_id to child tunnel from parent tunnel

The child tunnel if_id will be used for xfrm interface's lookup
when processing the IP(6)IP(6) packets in the next patches.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>