s4-dsdb: don't return the partition root objects

s4-dsdb: don't return the partition root objects

When searching across partitions, we want to avoid sending duplicate
records caused by the record appearing both as a mount point and as a
partition root in a nested partition. This patch works by intercepting
objects from searches and checking if they match a partition root. If
they do, and the partition is not the one in the partition control
request, then discard the object.