lib: ldb: Use NULL to allocate modules not talloc_autofree_context().

lib: ldb: Use NULL to allocate modules not talloc_autofree_context().

ldb modules are not (yet) unloaded and are only loaded once (there is a check
that makes sure of this). Allocate off the NULL context. We never want this
to be freed until process shutdown. If eventually we add the ability to
unload ldb modules we can add a deregister function that walks and frees the list.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12932

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>