btrfs: stop using write_one_page in btrfs_scratch_superblock

write_one_page is an awkward interface that expects the page locked and
->writepage to be implemented.  Replace that by zeroing the signature
bytes and synchronize the block device page using the proper bdev
helpers.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2018,23 +2018,22 @@ static void btrfs_scratch_superblock(struct btrfs_fs_info *fs_info,
 				     struct block_device *bdev, int copy_num)
 {
 	struct btrfs_super_block *disk_super;
-	struct page *page;
+	const size_t len = sizeof(disk_super->magic);
+	const u64 bytenr = btrfs_sb_offset(copy_num);
 	int ret;
 
-	disk_super = btrfs_read_dev_one_super(bdev, copy_num, false);
+	disk_super = btrfs_read_disk_super(bdev, bytenr, bytenr);
 	if (IS_ERR(disk_super))
 		return;
 
-	memset(&disk_super->magic, 0, sizeof(disk_super->magic));
-	page = virt_to_page(disk_super);
-	set_page_dirty(page);
-	lock_page(page);
-	/* write_on_page() unlocks the page */
-	ret = write_one_page(page);
+	memset(&disk_super->magic, 0, len);
+	folio_mark_dirty(virt_to_folio(disk_super));
+	btrfs_release_disk_super(disk_super);
+
+	ret = sync_blockdev_range(bdev, bytenr, bytenr + len - 1);
 	if (ret)
 		btrfs_warn(fs_info, "error clearing superblock number %d (%d)",
 			copy_num, ret);
-	btrfs_release_disk_super(disk_super);
 }
 
 void btrfs_scratch_superblocks(struct btrfs_fs_info *fs_info,