printing: add jobid_to_sysjob helper function

printing: add jobid_to_sysjob helper function

Samba needs to deal with two types of print job identifiers, those
allocated by the printing backend (sysjob ids), and those allocated
by Samba's spoolss server (jobids).

This change adds a helper function to map spoolss jobids to sysjob ids,
to go alongside the corresponding sysjob to jobid mapping function.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>