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>

net/core/filter.c

@@ -4119,6 +4119,10 @@ BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
 			sk->sk_sndbuf = max_t(int, val * 2, SOCK_MIN_SNDBUF);
 			break;
 		case SO_MAX_PACING_RATE: /* 32bit version */
+			if (val != ~0U)
+				cmpxchg(&sk->sk_pacing_status,
+					SK_PACING_NONE,
+					SK_PACING_NEEDED);
 			sk->sk_max_pacing_rate = (val == ~0U) ? ~0UL : val;
 			sk->sk_pacing_rate = min(sk->sk_pacing_rate,
 						 sk->sk_max_pacing_rate);