Avoid deadlock between orphan-truncate and cluster-write

Avoid deadlock between orphan-truncate and cluster-write

If, during truncate, we find an index block in writeback,
we need to give up on the orphan and return back up
to the cleaner thread so that the writeback can be
properly handled.
Otherwise the cleaner thread is blocked waiting for the cleaner thread
to make progress.

Signed-off-by: NeilBrown <neilb@suse.de>