perf stat: Clear evsel->reset_group for each stat run

If a weak group is broken then the reset_group flag remains set for
the next run. Having reset_group set means the counter isn't created
and ultimately a segfault.

A simple reproduction of this is:

  # perf stat -r2 -e '{cycles,cycles,cycles,cycles,cycles,cycles,cycles,cycles,cycles,cycles}:W

which will be added as a test in the next patch.

Fixes: 4804e011

 ("perf stat: Use affinity for opening events")
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20220822213352.75721-1-irogers@google.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>