rtnetlink: add IFLA_GROUP to ifla_policy

Network interface groups support added while ago, however
there is no IFLA_GROUP attribute description in policy
and netlink message size calculations until now.

Add IFLA_GROUP attribute to the policy.

Fixes: cbda10fa ("net_device: add support for network device groups")
Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>