rtnetlink: add IFLA_GROUP to ifla_policy

rtnetlink: add IFLA_GROUP to ifla_policy

BugLink: http://bugs.launchpad.net/bugs/1702863
[ Upstream commit db833d40ad3263b2ee3b59a1ba168bb3cfed8137 ]

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: cbda10fa97d7 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>