While at it: we can traverse ctx->group_list to get all
group leader, it should be safe since we hold ctx->mutex.

Changlog v1->v2:

  - remove WARN_ON_ONCE() according to Peter Zijlstra's suggestion
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <4ABC5AF9.6060808@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>