Impact: cleanup

Remove checking parent == NULL. It won't be NULLL, because we dynamically
create sub task_group only, and sub task_group always has its parent.
(root task_group is statically defined)

Also replace kmalloc_node(GFP_ZERO) with kzalloc_node().
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>