Btrfs: usage filter

Select chunks that are less than X percent full.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index fb20d77..eee77fc 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -200,6 +200,7 @@
  * Balance filters
  */
 #define BTRFS_BALANCE_ARGS_PROFILES	(1ULL << 0)
+#define BTRFS_BALANCE_ARGS_USAGE	(1ULL << 1)
 
 struct btrfs_balance_args;
 struct btrfs_balance_control {