perf bpf examples: Convert etcsnoop to use bpf_map()

perf bpf examples: Convert etcsnoop to use bpf_map()

Making the code more compact, end result is the same:

  # trace -e /home/acme/git/perf/tools/perf/examples/bpf/etcsnoop.c
     0.000 (         ): sed/7385 openat(dfd: CWD, filename: "/etc/ld.so.cache", flags: RDONLY|CLOEXEC) ...
  2727.723 (         ): cat/7389 openat(dfd: CWD, filename: "/etc/ld.so.cache", flags: RDONLY|CLOEXEC) ...
  2728.543 (         ): cat/7389 openat(dfd: CWD, filename: "/etc/passwd")                          ...
  ^C

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-znhgz24p0daux2kay200ovc1@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>