tcp: remove volatile qualifier on tw_substate

tcp: remove volatile qualifier on tw_substate

Using a volatile qualifier for a specific struct field is unusual.

Use instead READ_ONCE()/WRITE_ONCE() where necessary.

tcp_timewait_state_process() can change tw_substate while other
threads are reading this field.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20240827015250.3509197-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>