smbd: Simplify smbXsrv_open_global_store()

Avoid the dependency on global->db_rec. This makes the callers more
verbose, but it makes the data dependencies much more obvious. This
will enable removing smbXsrv_open_global0->db_rec at some point.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
