memcache: Properly track the size of talloc objects

memcache: Properly track the size of talloc objects

With memcache_add_talloc, the talloc object becomes part of the pool and
the memcache_element stores a pointer to the talloc object. The
size of the the talloc object was not used when tracking the used space,
allowing the cache to grow larger than defined in the memcache_init
call.

Fix this by adding the size of the talloc object to the used space.

Also record the initial size of the talloc object for proper adjustment
of the used space in the cache later. This is in case the size of the
talloc object is modified while being owned by the cache (e.g.
allocating talloc child objects). This should never happen, but better
be safe than ending up with a broken cache usage counter.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13865

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>