tcp: use dst_dev_rcu() in tcp_fastopen_active_disable_ofo_check()

Use RCU to avoid a pair of atomic operations and a potential
UAF on dst_dev()->flags.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250828195823.3958522-8-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>