ldb: One-level search was incorrectly falling back to full DB scan

ldb: One-level search was incorrectly falling back to full DB scan

When no search filter is specified, the code falls back to using
'(|(objectClass=*)(distinguishedName=*)'. ltdb_index_dn() then failed
because matching against '*' is not indexed. The error return then
caused the code to fallback to a full-scan of the DB, which could have a
considerable performance hit.

Instead, we want to continue on and do the ltdb_index_filter() over the
indexed results that were returned.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13448

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>