scrub_compaction assumes that `make_interposer_consumer()` is called
only when `use_interposer_consumer()` returns true. This is false, so in
effect scrub always ends up using the segregating interposer. Fix this
by short-circuiting the former method when the latter returns true,
returning the passed-in consumer unchanged.
Tests: unit(dev)
Fixes#9541Closes#9564