Btrfs: Enable btree balancing on old kernels again

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -1719,7 +1719,6 @@ void btrfs_btree_balance_dirty(struct btrfs_root *root, unsigned long nr)
 	 * looks as though older kernels can get into trouble with
 	 * this code, they end up stuck in balance_dirty_pages forever
 	 */
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,23)
 	struct extent_io_tree *tree;
 	u64 num_dirty;
 	u64 start = 0;
@@ -1735,9 +1734,7 @@ void btrfs_btree_balance_dirty(struct btrfs_root *root, unsigned long nr)
 		balance_dirty_pages_ratelimited_nr(
 				   root->fs_info->btree_inode->i_mapping, 1);
 	}
-#else
 	return;
-#endif
 }
 
 void btrfs_set_buffer_defrag(struct extent_buffer *buf)