Btrfs: fix btrfs_is_free_space_inode to recognize btree inode

For btree inode, its root is also 'tree root', so btree inode can be
misunderstood as a free space inode.

We should add one more check for btree inode.

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>