perf cgroup: Update metric leader in evlist__expand_cgroup

perf cgroup: Update metric leader in evlist__expand_cgroup

When the evlist is expanded the metric leader wasn't being updated. As
the original evsel is deleted this creates a use-after-free in
stat-shadow's prepare_metric. This was detected running the "perf stat
--bpf-counters --for-each-cgroup test" with sanitizers.

The change itself puts the copied evsel into the priv field (known
unused because of evsel__clone use) and then in a second pass over the
list updates the copied values using the priv pointer.

Fixes: d1c5a0e86a4e ("perf stat: Add --for-each-cgroup option")
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Sun Jian <sun.jian.kdev@gmail.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>