fq: allow options of fair queue set to ~0U

fq: allow options of fair queue set to ~0U

Some options of fair queue cannot be (~0U). It leads to maxrate
cannot be reset to unlimited because it cannot be (~0U). Allow
the options being ~0U.

Tested by the following command:
 # tc qdisc add dev eth4 root handle 1: fq limit 2000 flow_limit 200 maxrate 100mbit quantum 2000 initial_quantum 1600
 # tc -s -d qdisc show
qdisc fq 1: dev eth4 root refcnt 2 limit 2000p flow_limit 200p buckets 1024 quantum 2000 initial_quantum 1600 maxrate 100Mbit
 Sent 1492 bytes 10 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
  1 flows (0 inactive, 0 throttled)
  0 gc, 0 highprio, 0 throttled

 # tc qdisc change dev eth4 root handle 1: fq limit 4294967295 flow_limit 4294967295 maxrate 34359738360 quantum 4294967295 initial_quantum 4294967295
 # tc -s -d qdisc show
qdisc fq 1: dev eth4 root refcnt 2 limit 4294967295p flow_limit 4294967295p buckets 1024 quantum 4294967295 initial_quantum 4294967295
 Sent 38372 bytes 216 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
  2 flows (1 inactive, 0 throttled)
  0 gc, 2 highprio, 7 throttled

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>