unregister_blkdev(): return void

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: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b3a9f0d..aa74f7d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1527,7 +1527,7 @@
 
 #ifdef CONFIG_BLOCK
 extern int register_blkdev(unsigned int, const char *);
-extern int unregister_blkdev(unsigned int, const char *);
+extern void unregister_blkdev(unsigned int, const char *);
 extern struct block_device *bdget(dev_t);
 extern void bd_set_size(struct block_device *, loff_t size);
 extern void bd_forget(struct inode *inode);