Starting with a helper for a basic pid_map(), a hash using a pid as a
key.

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: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-gdwvq53wltvq6b3g5tdmh0cw@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>