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>

fs/btrfs/scrub.c

@@ -3678,7 +3678,7 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
 		 */
 		spin_lock(&cache->lock);
 		if (!cache->removed && !cache->ro && cache->reserved == 0 &&
-		    btrfs_block_group_used(&cache->item) == 0) {
+		    cache->used == 0) {
 			spin_unlock(&cache->lock);
 			btrfs_mark_bg_unused(cache);
 		} else {