bpf: fix SO_MAX_PACING_RATE to support TCP internal pacing

If sch_fq packet scheduler is not used, TCP can fallback to
internal pacing, but this requires sk_pacing_status to
be properly set.

Fixes: 8c4b4c7e ("bpf: Add setsockopt helper function to bpf")
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>