net-timestamp: namespacify the sysctl_tstamp_allow_data

Let it be tuned in per netns by admins.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20241005222609.94980-1-kerneljasonxing@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>