net_sched: avoid generating same handle for u32 filters

When kernel generates a handle for a u32 filter, it tries to start
from the max in the bucket. So when we have a filter with the max (fff)
handle, it will cause kernel always generates the same handle for new
filters. This can be shown by the following command:

	tc qdisc add dev eth0 ingress
	tc filter add dev eth0 parent ffff: protocol ip pref 770 handle 800::fff u32 match ip protocol 1 0xff
	tc filter add dev eth0 parent ffff: protocol ip pref 770 u32 match ip protocol 1 0xff
	...

we will get some u32 filters with same handle:

 # tc filter show dev eth0 parent ffff:
filter protocol ip pref 770 u32
filter protocol ip pref 770 u32 fh 800: ht divisor 1
filter protocol ip pref 770 u32 fh 800::fff order 4095 key ht 800 bkt 0
  match 00010000/00ff0000 at 8
filter protocol ip pref 770 u32 fh 800::fff order 4095 key ht 800 bkt 0
  match 00010000/00ff0000 at 8
filter protocol ip pref 770 u32 fh 800::fff order 4095 key ht 800 bkt 0
  match 00010000/00ff0000 at 8
filter protocol ip pref 770 u32 fh 800::fff order 4095 key ht 800 bkt 0
  match 00010000/00ff0000 at 8

handles should be unique. This patch fixes it by looking up a bitmap,
so that can guarantee the handle is as unique as possible. For compatibility,
we still start from 0x800.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>