perf: Make clock software events consistent with general exclusion rules

The cpu/task clock events implement their own version of exclusion
on top of exclude_user and exclude_kernel.

The result is that when the event triggered in the kernel but we
have exclude_kernel set, we try to rewind using task_pt_regs.
There are two side effects of this:

- we call task_pt_regs even on kernel threads, which doesn't give
  us the desired result.
- if the event occured in the kernel, we shouldn't rewind to the
  user context. We want to actually ignore the event.

get_irq_regs() will always give us the right interrupted context, so
use its result and submit it to perf_exclude_context() that knows
when an event must be ignored.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>