net: bridge: vlan: add new rtm message support

Add initial RTM_NEWVLAN support which can only create vlans, operating
similar to the current br_afspec(). We will use it later to also change
per-vlan options. Old-style (flag-based) vlan ranges are not allowed
when using RTM messages, we will introduce vlan ranges later via a new
nested attribute which would allow us to have all the information about a
range encapsulated into a single nl attribute.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>