htb: support 64bit rates

htb: support 64bit rates

Starting from linux-3.13, we can break the 32bit limitation of
rates on HTB qdisc/classes.

Prior limit was 34.359.738.360 bits per second.

lpq83:~# tc -s qdisc show dev lo ; tc -s class show dev lo
qdisc htb 1: root refcnt 2 r2q 2000 default 1 direct_packets_stat 0 direct_qlen 6000
 Sent 6591936144493 bytes 149549182 pkt (dropped 0, overlimits 213757419 requeues 0)
 rate 39464Mbit 114938pps backlog 0b 15p requeues 0
class htb 1:1 root prio 0 rate 50000Mbit ceil 50000Mbit burst 200000b cburst 0b
 Sent 6591942184547 bytes 149549310 pkt (dropped 0, overlimits 0 requeues 0)
 rate 39464Mbit 114938pps backlog 0b 15p requeues 0
 lended: 149549310 borrowed: 0 giants: 0
 tokens: 336 ctokens: -164

Signed-off-by: Eric Dumazet <edumazet@google.com>