tc: Add support for the CBS qdisc

tc: Add support for the CBS qdisc

The Credit Based Shaper (CBS) queueing discipline allows bandwidth
reservation with sub-milisecond precision. It is defined by the
802.1Q-2014 specification (section 8.6.8.2 and Annex L).

The syntax is:

tc qdisc add dev DEV parent NODE cbs locredit <LOCREDIT>
   		hicredit <HICREDIT> sendslope <SENDSLOPE>
		idleslope <IDLESLOPE>

(The order is not important)

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>