tcp_metrics: annotate data-races around tm->tcpm_vals[]

[ Upstream commit 8c4d04f6b443869d25e59822f7cec88d647028a9 ]

tm->tcpm_vals[] values can be read or written locklessly.

Add needed READ_ONCE()/WRITE_ONCE() to document this,
and force use of tcp_metric_get() and tcp_metric_set()

Fixes: 51c5d0c4 ("tcp: Maintain dynamic metrics in local cache.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>