net-netlink: Add a new attribute to expose TCLASS values via netlink

commit 3ceca749668a52bd795585e0f71c6f0b04814f7b added a TOS attribute.

Unfortunately TOS and TCLASS are both present in a dual-stack v6 socket,
furthermore they can have different values.  As such one cannot in a
sane way expose both through a single attribute.

Signed-off-by: Maciej Żenczyowski <maze@google.com>
CC: Murali Raja <muralira@google.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>