dcb: Add a subtool for the DCB buffer object

dcb: Add a subtool for the DCB buffer object

DCBNL buffer interfaces are an extension to the 802.1q DCB interfaces and
allow configuration of port headroom buffers.

Add a dcb subtool to allow showing and tweaking of buffer priority mapping
and buffer sizes. For example:

    # dcb buf show dev eni1np1
    prio-buffer 0:0 1:0 2:0 3:3 4:0 5:0 6:6 7:0
    buffer-size 0:10000 1:0 2:0 3:70000 4:0 5:0 6:10000 7:0
    total-size 221072

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