perf cgroup: Introduce cgroup__get()

The refcount operation counterpart to cgroup__put(), use it when reusing
a cgroup.

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-14ynvrl7y2cz8gyuy5q5v41g@git.kernel.org


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