talloc: Decouple the dual use of chunk->pool

talloc: Decouple the dual use of chunk->pool

If we want nested pools, we will have pools that are pool members. So
we will have to have a separate "next object" pointer  for pools. As
we have struct talloc_pool_chunk now, this additional pointer does not
affect normal talloc objects.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 1334c745e1f2157b66e14f9d8b4f6f7750238717)