Cherry-pick fix to break out of loop in cgroup_attach_task()

Manual pick of

[PATCH] cgroup: fix to break the while loop in cgroup_attach_task() correctly

This makes cts content resolver test not freeze the emulator every once
in a while.

Change-Id: I9ef552d5ae24296e7898ce5c93abfaa9d90c241c
Signed-off-by: Lingfeng Yang <lfy@google.com>