block/md: fix md autodetection

Block ext devt conversion missed md_autodetect_dev() call in
rescan_partitions() leaving md autodetect unable to see partitions.
Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>