idmap: Prime gencache after xids2sids calls

idmap: Prime gencache after xids2sids calls

This fixes a performance regression for "hide unreadable". With an empty
gencache, we only do xid2sid calls when reading a large number of acls. We
lost caching the xid2sid calls while implmenting the multiple-id calls,
probably because at that time the bug with ID_TYPE_BOTH backends was still
pending. This patch restores the xid2sid caching hopefully correctly.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12484

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Uri Simchoni <uri@samba.org>