smbd: only reprocess printer_list.tdb if it changed

smbd: only reprocess printer_list.tdb if it changed

The per-client smbd printer share inventory is currently updated from
printer_list.tdb when a client enumerates printers, via EnumPrinters or
NetShareEnum.
printer_list.tdb is populated by the background print process, based on
the latest printcap values retrieved from the printing backend (e.g.
CUPS) at regular intervals.
This change ensures that per-client smbd processes don't reparse
printer_list.tdb if it hasn't been updated since the last enumeration.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10652

Suggested-by: Volker Lendecke <vl@samba.org>
Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>