btrfs: factor out scratching of one regular super block

btrfs_scratch_superblocks open codes scratching super block of a
non-zoned super block.  Split the code to read, zero and write the
superblock for regular devices into a separate helper.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
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

@@ -2014,42 +2014,43 @@ static u64 btrfs_num_devices(struct btrfs_fs_info *fs_info)
 	return num_devices;
 }
 
+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;
+	int ret;
+
+	disk_super = btrfs_read_dev_one_super(bdev, copy_num, false);
+	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);
+	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,
 			       struct block_device *bdev,
 			       const char *device_path)
 {
-	struct btrfs_super_block *disk_super;
 	int copy_num;
 
 	if (!bdev)
 		return;
 
 	for (copy_num = 0; copy_num < BTRFS_SUPER_MIRROR_MAX; copy_num++) {
-		struct page *page;
-		int ret;
-
-		disk_super = btrfs_read_dev_one_super(bdev, copy_num, false);
-		if (IS_ERR(disk_super))
-			continue;
-
-		if (bdev_is_zoned(bdev)) {
+		if (bdev_is_zoned(bdev))
 			btrfs_reset_sb_log_zones(bdev, copy_num);
-			continue;
-		}
-
-		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);
-		if (ret)
-			btrfs_warn(fs_info,
-				"error clearing superblock number %d (%d)",
-				copy_num, ret);
-		btrfs_release_disk_super(disk_super);
-
+		else
+			btrfs_scratch_superblock(fs_info, bdev, copy_num);
 	}
 
 	/* Notify udev that device has changed */