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

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230719212857.3943972-5-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>