dcb: rewr: add new dcb-rewr subcommand

dcb: rewr: add new dcb-rewr subcommand

Add a new subcommand 'rewr' for configuring the in-kernel DCB rewrite
table. The rewrite table of the kernel is similar to the APP table,
therefore, much of the existing bookkeeping code from dcb-app, can be
reused in the dcb-rewr implementation.

Initially, only support for configuring PCP and DSCP-based rewrite has
been added.

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