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

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.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>