Btrfs: kill root from btrfs_is_free_space_inode

Since root can be fetched via BTRFS_I macro directly, we can save an args
for btrfs_is_free_space_inode().

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