ldb_tdb: replace strange dn_list_cmp() in index code

ldb_tdb: replace strange dn_list_cmp() in index code

This replaces dn_list_cmp() with functions that do not attempt to
to care about string termination.  All index values are case sensitive
and correctly length-bound already, even for a DN index
so just use a length check and memcmp()

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