Commit 592b09a4 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: NArtur Skawina <art.08.09@gmail.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>