perf evsel: Introduce method to request IDs be used

When mmaping multiple events we need to find the right evsel that
matches an event in the ring buffer.

For that we need to set the PERF_FORMAT_ID bit in
perf_event_attr.read_format so that when we read the event fds we get
that id to then hash it and be able later to use perf_evlist__id2evsel
to find the right evsel.

We also need to set the PERF_SAMPLE_ID bit in
perf_event_attr.sample_type to ask for that id to be stashed in each
sample, so that we can demux it.

So add a perf_evsel__set_sample_id() method to do those two things in
one operation.

Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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