I got a segfault when using -r option with event groups.  The option
makes it run the workload multiple times and it will reuse the evlist
and evsel for each run.

While most of resources are allocated and freed properly, the id hash
in the evlist was not and it resulted in the bug.  You can see it with
the address sanitizer like below:

  $ perf stat -r 100 -e '{cycles,instructions}' true
  =================================================================
  ==693052==ERROR: AddressSanitizer: heap-use-after-free on
      address 0x6080000003d0 at pc 0x558c57732835 bp 0x7fff1526adb0 sp 0x7fff1526ada8
  WRITE of size 8 at 0x6080000003d0 thread T0
    #0 0x558c57732834 in hlist_add_head /home/namhyung/project/linux/tools/include/linux/list.h:644
    #1 0x558c57732834 in perf_evlist__id_hash /home/namhyung/project/linux/tools/lib/perf/evlist.c:237
    #2 0x558c57732834 in perf_evlist__id_add /home/namhyung/project/linux/tools/lib/perf/evlist.c:244
    #3 0x558c57732834 in perf_evlist__id_add_fd /home/namhyung/project/linux/tools/lib/perf/evlist.c:285
    #4 0x558c5747733e in store_evsel_ids util/evsel.c:2765
    #5 0x558c5747733e in evsel__store_ids util/evsel.c:2782
    #6 0x558c5730b717 in __run_perf_stat /home/namhyung/project/linux/tools/perf/builtin-stat.c:895
    #7 0x558c5730b717 in run_perf_stat /home/namhyung/project/linux/tools/perf/builtin-stat.c:1014
    #8 0x558c5730b717 in cmd_stat /home/namhyung/project/linux/tools/perf/builtin-stat.c:2446
    #9 0x558c57427c24 in run_builtin /home/namhyung/project/linux/tools/perf/perf.c:313
    #10 0x558c572b1a48 in handle_internal_command /home/namhyung/project/linux/tools/perf/perf.c:365
    #11 0x558c572b1a48 in run_argv /home/namhyung/project/linux/tools/perf/perf.c:409
    #12 0x558c572b1a48 in main /home/namhyung/project/linux/tools/perf/perf.c:539
    #13 0x7fcadb9f7d09 in __libc_start_main ../csu/libc-start.c:308
    #14 0x558c572b60f9 in _start (/home/namhyung/project/linux/tools/perf/perf+0x45d0f9)

Actually the nodes in the hash table are struct perf_stream_id and
they were freed in the previous run.  Fix it by resetting the hash.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20210225035148.778569-2-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>