Put WARN_ON and fixed all callers of unregister_blkdev().  Now we can make
unregister_blkdev return void.

Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>