Btrfs: fix memory leak of empty filesystem after balance

Btrfs: fix memory leak of empty filesystem after balance

After Josef's patch(commit 3fd19cc0795104f010a85c250d579103fb7a5b36),
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

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>