net_sched: sch_pie: implement lockless pie_dump()

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

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>