freezer_cg: simplify freezer_change_state()

Just call unfreeze_cgroup() if goal_state == THAWED, and call
try_to_freeze_cgroup() if goal_state == FROZEN.

No behavior has been changed.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Cedric Le Goater <clg@fr.ibm.com>
Acked-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>