perf cgroup: Clean up perf_cgroup_create()

- Use kzalloc() to replace kmalloc() + memset().

- Remove redundant initialization, since alloc_percpu() returns
  zero-filled percpu memory.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <4D6F347E.2010806@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>