umount: avoid racy block-size set

Setting the block size back to the original blocksize at umount time
is unnecessary, and can cause buffer errors and infinite loops in
__getblk_slow, and possibly other problems. 

Just getting bdev_lock instead causes other issues, namely AB-BA deadlocks
with /proc/meminfo etc. Just drop it, and replace with a "sync_blockdev()"
to cause a flush (which was a side effect of changing the blocksize before).

Debugging and testing by Jeff Mahoney <jeffm@suse.com>

fs/super.c

@@ -732,7 +732,7 @@ void kill_block_super(struct super_block *sb)
 
 	bdev_uevent(bdev, KOBJ_UMOUNT);
 	generic_shutdown_super(sb);
-	set_blocksize(bdev, sb->s_old_blocksize);
+	sync_blockdev(bdev);
 	close_bdev_excl(bdev);
 }