tcp: annotate data-races around tp->tsoffset

do_tcp_getsockopt() reads tp->tsoffset while another cpu
might change its value.

Fixes: 93be6ce0

 ("tcp: set and get per-socket timestamp")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230719212857.3943972-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>