perf metricgroup: Use strsep()

No change in behaviour intended, trivial optimization done by avoiding
looking for spaces in 'g' right after setting it to "No_group".

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-f2siadtp3hb5o0l1w7bvd8bk@git.kernel.org


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