list: add list_for_each_rev_off macro

And re-implement list_for_each_rev in terms of list_for_each_rev_off
to avoid duplicating iteration logic.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>