perf metricgroup: Fix system PMU metrics

Joakim reports that getting "perf stat" for multiple system PMU metrics
segfaults:

  $ perf stat -a -I 1000 -M imx8mm_ddr_write.all,imx8mm_ddr_write.all
  Segmentation fault
  $

While the same works without issue for a single metric.

The logic in metricgroup__add_metric_sys_event_iter() is broken, in that
add_metric() @m argument should be NULL for each new metric. Fix by not
passing a holder for that, and rather make local in
metricgroup__add_metric_sys_event_iter().

Fixes: be335ec2

 ("perf metricgroup: Support adding metrics for system PMUs")
Reported-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linuxarm@openeuler.org
Link: https://lore.kernel.org/r/1611050655-44020-1-git-send-email-john.garry@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>