perf: add the necessary core perf APIs when accessing events counters in eBPF programs

This patch add three core perf APIs:
 - perf_event_attrs(): export the struct perf_event_attr from struct
   perf_event;
 - perf_event_get(): get the struct perf_event from the given fd;
 - perf_event_read_local(): read the events counters active on the
   current CPU;
These APIs are needed when accessing events counters in eBPF programs.

The API perf_event_read_local() comes from Peter and I add the
corresponding SOB.

Signed-off-by: Kaixu Xia <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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