dcb: add new subcommand for apptrust

dcb: add new subcommand for apptrust

Add new apptrust subcommand for the dcbnl apptrust extension object.

The apptrust command lets you specify a consecutive ordered list of
trusted selectors, which can be used by drivers to determine which
selectors are eligible (trusted) for packet prioritization, and in which
order.

Selectors are sent in a new nested attribute:
DCB_ATTR_IEEE_APP_TRUST_TABLE.  The nest contains trusted selectors
encapsulated in either DCB_ATTR_IEEE_APP or DCB_ATTR_DCB_APP attributes,
for standard and non-standard selectors, respectively.

Example:

Trust selectors dscp and pcp, in that order
$ dcb apptrust set dev eth0 order dscp pcp

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>