Btrfs: add balance progress reporting

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index caa9abd..6faec9d 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -218,6 +218,7 @@
 #define BTRFS_BALANCE_ARGS_SOFT		(1ULL << 9)
 
 struct btrfs_balance_args;
+struct btrfs_balance_progress;
 struct btrfs_balance_control {
 	struct btrfs_fs_info *fs_info;
 
@@ -226,6 +227,8 @@
 	struct btrfs_balance_args sys;
 
 	u64 flags;
+
+	struct btrfs_balance_progress stat;
 };
 
 int btrfs_account_dev_extents_size(struct btrfs_device *device, u64 start,