net_sched: cake: implement lockless cake_dump()

Instead of relying on RTNL, cake_dump() can use READ_ONCE()
annotations, paired with WRITE_ONCE() ones in cake_change().

v2: addressed Simon feedback in V1: https://lore.kernel.org/netdev/20240417083549.GA3846178@kernel.org/

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Toke Høiland-Jørgensen <toke@toke.dk>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>