Btrfs: fix memory leak of empty filesystem after balance

After Josef's patch(commit 3c14874acc71180553fb5aba528e3cf57c5b958b), btrfs will exclude super bytes when reading block groups(by marking a extent state UPTODATE). However, these bytes do not get freed while balance remove unused block groups, and we won't process those removed ones any more, when we do umount and unload the btrfs module, btrfs hits a memory leak.

This patch add the missing free operation.

Reproduce steps:

$ mkfs.btrfs disk
$ mount disk /mnt/btrfs -o loop
$ btrfs filesystem balance /mnt/btrfs
$ umount /mnt/btrfs
$ rmmod btrfs

---
 fs/btrfs/extent-tree.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

index 08525ee..a1af67a 100644
@@ -8611,6 +8611,12 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
 	BUG_ON(!block_group);
 	BUG_ON(!block_group->ro);
 
+	/*
+	 * Free the reserved super bytes from this block group before
+	 * remove it.
+	 */
+	free_excluded_extents(root, block_group);
+
 	memcpy(&key, &block_group->key, sizeof(key));
 	if (block_group->flags & (BTRFS_BLOCK_GROUP_DUP |
 				  BTRFS_BLOCK_GROUP_RAID1 |

1.6.5.2