bpf: tcp: Allow to write tp->snd_cwnd_stamp in bpf_tcp_ca

This patch allows the write of tp->snd_cwnd_stamp in a bpf tcp
ca program. An use case of writing this field is to keep track
of the time whenever tp->snd_cwnd is raised or reduced inside
the `cong_control` callback.
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Miao Xu <miaxu@meta.com>
Link: https://lore.kernel.org/r/20240502042318.801932-3-miaxu@meta.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

net/ipv4/bpf_tcp_ca.c

@@ -107,6 +107,9 @@ static int bpf_tcp_ca_btf_struct_access(struct bpf_verifier_log *log,
 	case offsetof(struct tcp_sock, snd_cwnd_cnt):
 		end = offsetofend(struct tcp_sock, snd_cwnd_cnt);
 		break;
+	case offsetof(struct tcp_sock, snd_cwnd_stamp):
+		end = offsetofend(struct tcp_sock, snd_cwnd_stamp);
+		break;
 	case offsetof(struct tcp_sock, snd_ssthresh):
 		end = offsetofend(struct tcp_sock, snd_ssthresh);
 		break;