replmd: Make replmd_set_la_val() closer to replmd_build_la_val()

replmd: Make replmd_set_la_val() closer to replmd_build_la_val()

These two functions are almost identical. The main difference between
them is the RMD_ADDTIME. replmd_set_la_val() tries to use the
RMD_ADDTIME of the old_dsdb_dn. Whereas replmd_build_la_val() always
uses the time passed in.

Change replmd_set_la_val() so it can accept a NULL old_dsdb_dn (i.e. if
it's a new linked attribute that's being set). If so, it'll end up using
the nttime parameter passed in, same as replmd_build_la_val() does.

Also update replmd_process_linked_attribute (which used to use
replmd_build_la_val()) to now pass in a NULL old_dsdb_dn. There
shouldn't be a difference in behaviour either way, but this exercises
the code change.

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>