QUIC servers would like to use SO_TXTIME, without having CAP_NET_ADMIN,
to efficiently pace UDP packets.

As far as sch_fq is concerned, we need to add safety checks, so
that a buggy application does not fill the qdisc with packets
having delivery time far in the future.

This patch adds a configurable horizon (default: 10 seconds),
and a configurable policy when a packet is beyond the horizon
at enqueue() time:
- either drop the packet (default policy)
- or cap its delivery time to the horizon.

$ tc -s -d qd sh dev eth0
qdisc fq 8022: root refcnt 257 limit 10000p flow_limit 100p buckets 1024
 orphan_mask 1023 quantum 10Kb initial_quantum 51160b low_rate_threshold 550Kbit
 refill_delay 40.0ms timer_slack 10.000us horizon 10.000s
 Sent 1234215879 bytes 837099 pkt (dropped 21, overlimits 0 requeues 6)
 backlog 0b 0p requeues 6
  flows 1191 (inactive 1177 throttled 0)
  gc 0 highprio 0 throttled 692 latency 11.480us
  pkts_too_long 0 alloc_errors 0 horizon_drops 21 horizon_caps 0

v2: fixed an overflow on 32bit kernels in fq_init(), reported
    by kbuild test robot <lkp@intel.com>

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>