backing-dev: bdi sb prune should be in the unregister path, not destroy

backing-dev: bdi sb prune should be in the unregister path, not destroy

Commit 2df49383e2e2a23cd6ee954d036a51d25728c60f was different from
the tested path, in that it moved the bdi super_block prune from
unregister to destroy context. This doesn't fully fix the sync hang
bug on unexpected device removal, as need to prune the bdi cache
pointer before killing flusher thread.

Tested-by: Artur Skawina <art.08.09@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>