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 <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-1z4xcmbud30lamklfe80oopu@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>