selftests/bpf: extend BPF_SOCK_OPS_RTT_CB test for srtt and mrtt_us

selftests/bpf: extend BPF_SOCK_OPS_RTT_CB test for srtt and mrtt_us

Because srtt and mrtt_us are added as args in bpf_sock_ops at
BPF_SOCK_OPS_RTT_CB, a simple check is added to make sure they are both
non-zero.

$ ./test_progs -t tcp_rtt
  #373     tcp_rtt:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Suggested-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240425161724.73707-3-lulie@linux.alibaba.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>