ldb: Rename variable

The old name confused me because it's not really related to the
one-level index at all. It's the result from evaluating the indexed
search specified in the ac->tree.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13762

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