perf cgroup: Introduce cgroup__new() out of open coded equivalent

To follow the namespacing convention in tools/perf.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-jaalyl6bkvvji4r5u8wqw4n4@git.kernel.org


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