btrfs: btrfs_rm_device() should zero mirror SB as well

	 * remove it from the devices list and zero out the old super

	 */

	if (clear_super && disk_super) {

		u64 bytenr;

		int i;

		/* make sure this device isn't detected as part of

		 * the FS anymore

		 */

		memset(&disk_super->magic, 0, sizeof(disk_super->magic));

		set_buffer_dirty(bh);

		sync_dirty_buffer(bh);

		/* clear the mirror copies of super block on the disk

		 * being removed, 0th copy is been taken care above and

		 * the below would take of the rest

		 */

		for (i = 1; i < BTRFS_SUPER_MIRROR_MAX; i++) {

			bytenr = btrfs_sb_offset(i);

			if (bytenr + BTRFS_SUPER_INFO_SIZE >=

					i_size_read(bdev->bd_inode))

				break;

			brelse(bh);

			bh = __bread(bdev, bytenr / 4096,

					BTRFS_SUPER_INFO_SIZE);

			if (!bh)

				continue;

			disk_super = (struct btrfs_super_block *)bh->b_data;

			if (btrfs_super_bytenr(disk_super) != bytenr ||

				btrfs_super_magic(disk_super) != BTRFS_MAGIC) {

				continue;

			}

			memset(&disk_super->magic, 0,

						sizeof(disk_super->magic));

			set_buffer_dirty(bh);

			sync_dirty_buffer(bh);

		}

	}

	ret = 0;