ripd, ripngd: fix cleaning up of offset lists

ripd, ripngd: fix cleaning up of offset lists

We should never attempt to remove a list item in the "del" callback
of the list. This is already performed by the list_delete() function,
doing it twice leads to crashes or memory corruption.

Introduce the offset_list_free() function so that we can separate the
removal and deallocation of offset lists into separate functions,
without code duplication. offset_list_del() will be used by the
northbound callbacks to remove offset lists, while offset_list_free()
will be used by rip_clean() to clean up all RIP offset lists using
list_delete(). Do the same for ripngd.

This is a fallout from the ripd/ripngd northbound conversion.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>