perf stat: Rename perf_stat struct into perf_stat_evsel

It's used as the perf_evsel::priv data, so the name suits better. Also
we'll need the perf_stat name free for more generic struct.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kan Liang <kan.liang@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1444992092-17897-29-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>