ldb_tdb: Do not care about duplicates if single value check disabled

ldb_tdb: Do not care about duplicates if single value check disabled

This behaviour of ignoring duplicates with the flag
LDB_FLAG_INTERNAL_DISABLE_SINGLE_VALUE_CHECK is also used in the replace
case here.

When we add a forward DN+Binary link with a duplicate DN, this prevents
us from not being able to add the backlink because it appears to be a
duplicate here.

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Pair-programmed-with: Bob Campbell <bobcampbell@catalyst.net.nz>