btrfs-progs: mkfs: do not use extent buffer to write super block

There is a long history that we used extent_buffer helpers to modify a
superblock, but we're moving out of that behavior, and migrate to use
the on stack helpers to modify super blocks.

In the function make_btrfs() we use a dummy extent buffer just to
calculate the checksum for the super block.
We do not need such workaround, and can manually call btrfs_csum_data()
on @super, and pass @super directly to sbwrite(), completely avoiding the
need of a dummy extent buffer.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

mkfs/common.c

@@ -754,12 +754,9 @@ int make_btrfs(int fd, struct btrfs_mkfs_config *cfg)
 	}
 
 	/* and write out the super block */
-	memset(buf->data, 0, BTRFS_SUPER_INFO_SIZE);
-	memcpy(buf->data, &super, sizeof(super));
-	buf->len = BTRFS_SUPER_INFO_SIZE;
-	csum_tree_block_size(buf, btrfs_csum_type_size(cfg->csum_type), 0,
-			     cfg->csum_type);
-	ret = sbwrite(fd, buf->data, BTRFS_SUPER_INFO_OFFSET);
+	btrfs_csum_data(cfg->csum_type, (u8 *)&super + BTRFS_CSUM_SIZE,
+			super.csum, sizeof(super) - BTRFS_CSUM_SIZE);
+	ret = sbwrite(fd, &super, BTRFS_SUPER_INFO_OFFSET);
 	if (ret != BTRFS_SUPER_INFO_SIZE) {
 		ret = (ret < 0 ? -errno : -EIO);
 		goto out;