perf: Cure task_oncpu_function_call() races

Oleg reported that on architectures with
__ARCH_WANT_INTERRUPTS_ON_CTXSW the IPI from
task_oncpu_function_call() can land before perf_event_task_sched_in()
and cause interesting situations for eg. perf_install_in_context().

This patch reworks the task_oncpu_function_call() interface to give a
more usable primitive as well as rework all its users to hopefully be
more obvious as well as remove the races.

While looking at the code I also found a number of races against
perf_event_task_sched_out() which can flip contexts between tasks so
plug those too.

Reported-and-reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -2578,13 +2578,6 @@ static inline void inc_syscw(struct task_struct *tsk)
 #define TASK_SIZE_OF(tsk)	TASK_SIZE
 #endif
 
-/*
- * Call the function if the target task is executing on a CPU right now:
- */
-extern void task_oncpu_function_call(struct task_struct *p,
-				     void (*func) (void *info), void *info);
-
-
 #ifdef CONFIG_MM_OWNER
 extern void mm_update_next_owner(struct mm_struct *mm);
 extern void mm_init_owner(struct mm_struct *mm, struct task_struct *p);