perf cgroup: Add evlist__findnew_cgroup()

perf cgroup: Add evlist__findnew_cgroup()

Similar to machine__findnew_thread(), etc, i.e. try to find, get a
refcount if found and return it, otherwise return a new cgroup object.

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-im1omevlihhyneiic4nl3g24@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>