perf: Introduce perf_pmu_migrate_context()

Originally from Peter Zijlstra. The helper migrates perf events
from one cpu to another cpu.

Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1339741902-8449-5-git-send-email-zheng.z.yan@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --cc include/linux/perf_event.h
Simple merge

diff --cc kernel/events/core.c
Simple merge