tcp: annotate data-races around tp->rx_opt.user_mss

This field is already read locklessly for listeners,
next patch will make setsockopt(TCP_MAXSEG) lockless.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20250821141901.18839-2-edumazet@google.com


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