perf test bpf: Use designated struct field initializers

perf test bpf: Use designated struct field initializers

To follow standard practice in the kernel sources, documenting the
initialization better and helping quickly finding the value for some
field in a struct with many entries.

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-syn3hz9hz7ukxlxbx5x6hv20@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>