net_sched: sch_choke: implement lockless choke_dump()

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

v2: added a WRITE_ONCE(p->Scell_log, Scell_log)
    per Simon feedback in V1
    Removed the READ_ONCE(q->limit) in choke_enqueue()

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