replmd: Partial fix for single-valued link conflict

replmd: Partial fix for single-valued link conflict

This is the first part of the fix for resolving a single-valued link
conflict.

When processing the replication data for a linked attribute, if we don't
find a match for the link target value, check if the link is a
single-valued attribute and it currently has an active link. If so, then
use the active link instead.

This change means we delete the existing active link (and backlink)
before adding the new link. This prevents the failure in the subsequent
dsdb_check_single_valued_link() check that was happening previously
(because the link would end up with 2 active values).

This is only a partial fix. It stops replication from failing completely
if we ever hit this situation (which means the test is no longer
hitting an assertion when replicating). However, ideally the existing
active link should be retained and just marked as deleted (with this
change, the existing link is overwritten completely).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13055

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