btrfs: move block_group_item::used to block group

For unknown reasons, the member 'used' in the block group struct is
stored in the b-tree item and accessed everywhere using the special
accessor helper. Let's unify it and make it a regular member and only
update the item before writing it to the tree.

The item is still being used for flags and chunk_objectid, there's some
duplication until the item is removed in following patches.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 98979dd..8e27633 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -404,7 +404,7 @@ static ssize_t raid_bytes_show(struct kobject *kobj,
 		if (&attr->attr == BTRFS_ATTR_PTR(raid, total_bytes))
 			val += block_group->key.offset;
 		else
-			val += btrfs_block_group_used(&block_group->item);
+			val += block_group->used;
 	}
 	up_read(&sinfo->groups_sem);
 	return snprintf(buf, PAGE_SIZE, "%llu\n", val);