perf_events: Fix rcu and locking issues with cgroup support

This patches ensures that we do not end up calling
perf_cgroup_from_task() when there is no cgroup event.
This avoids potential RCU and locking issues.

The change in perf_cgroup_set_timestamp() ensures we
check against ctx->nr_cgroups. It also avoids calling
perf_clock() tiwce in a row. It also ensures we do need
to grab ctx->lock before calling the function.

We drop update_cgrp_time() from task_clock_event_read()
because it is not needed. This also avoids having to
deal with perf_cgroup_from_task().

Thanks to Peter Zijlstra for his help on this.

Signed-off-by: Stephane Eranian <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

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