tcp: annotate data-races around fastopenq.max_qlen

[ Upstream commit 70f360dd ]

This field can be read locklessly.

Fixes: 1536e285

 ("tcp: Add a TCP_FASTOPEN socket option to get a max backlog on its listner")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230719212857.3943972-12-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>