tm->tcpm_lock can be read or written locklessly.

Add needed READ_ONCE()/WRITE_ONCE() to document this.

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>
Link: https://lore.kernel.org/r/20230802131500.1478140-4-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>