repl_md: Avoid dropping cross-partition links

repl_md: Avoid dropping cross-partition links

Cross-partition links could still be dropped if GET_TGT was already
previously set for the replication.

This was due to a slight error in the order of logic. We never want to
ignore cross-partition links (regardless of whether the TARGETS_UPTODATE
/GET_TGT flag is set). We should only be returning early in the
GET_TGT case if the objects are both in the same partition.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14022
RN: When the AD domain contained a linked attribute that spanned
partitions, DRS replication could drop the link. This dropped link could
then result in subtle differences in behaviour between DCs, as some DCs
would have the link and others wouldn't. When this issue occurred, the
dropped link would be logged in a warning message:
 "<target-dn> is Unknown but up to date. Ignoring link from <source-dn>"
This issue would not always occur - it depended a lot on the database
contents. Typically, it would only potentially occur when joining a new
DC to the domain (doing an ldapcmp after the join would also highlight
the problem, if it occurred). This issue has now been resolved.

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