vxlan: Add missing entries to vxlan_get_size()

There are some attributes added by vxlan_fill_info() which are not
accounted for in vxlan_get_size(). Add them.

I didn't find a way to trigger an actual problem from this miscalculation
since there is usually extra space in netlink size calculations like
if_nlmsg_size(); but maybe I just didn't search long enough.

Fixes: 3511494c ("vxlan: Group Policy extension")
Fixes: e1e5314d ("vxlan: implement GPE")
Fixes: 0ace2ca8 ("vxlan: Use checksum partial with remote checksum offload")
Fixes: f9c4bb0b

 ("vxlan: vni filtering support on collect metadata device")
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>