TCQ_F_CAN_BYPASS can be used by few qdiscs.

Idea is that if we queue a packet to an empty qdisc,
following dequeue() would pick it immediately.

FQ can not use the generic TCQ_F_CAN_BYPASS code,
because some additional checks need to be performed.

This patch adds a similar fast path to FQ.

Most of the time, qdisc is not throttled,
and many packets can avoid bringing/touching
at least four cache lines, and consuming 128bytes
of memory to store the state of a flow.

After this patch, netperf can send UDP packets about 13 % faster,
and pktgen goes 30 % faster (when FQ is in the way), on a fast NIC.

TCP traffic is also improved, thanks to a reduction of cache line misses.
I have measured a 5 % increase of throughput on a tcp_rr intensive workload.

tc -s -d qd sh dev eth1
...
qdisc fq 8004: parent 1:2 limit 10000p flow_limit 100p buckets 1024
   orphan_mask 1023 quantum 3028b initial_quantum 15140b low_rate_threshold 550Kbit
   refill_delay 40ms timer_slack 10us horizon 10s horizon_drop
 Sent 5646784384 bytes 1985161 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
  flows 122 (inactive 122 throttled 0)
  gc 0 highprio 0 fastpath 659990 throttled 27762 latency 8.57us
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>