dcb: Add a subtool for the DCB maxrate object

dcb: Add a subtool for the DCB maxrate object

DCBNL maxrate interfaces are an extension to the 802.1q DCB interfaces and
allow configuration of rate with which traffic in a given traffic class is
sent.

Add a dcb subtool to allow showing and tweaking of this per-TC maximum
rate. For example:

    # dcb maxrate show dev eni1np1
    tc-maxrate 0:25Gbit 1:25Gbit 2:25Gbit 3:25Gbit 4:25Gbit 5:25Gbit 6:100Gbit 7:25Gbit

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@gmail.com>