perf bpf: Convert pid_map() to bpf_map()

perf bpf: Convert pid_map() to bpf_map()

First user, pid_t as the type, lets see how this goes with the BTF
routines.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-56eplvf86r69wt3p35nh805z@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>