bpf: add a test case for helper bpf_perf_event_read_value

The bpf sample program tracex6 is enhanced to use the new
helper to read enabled/running time as well.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>