rtnetlink: fix rtnl_vfinfo_size

The size reported by rtnl_vfinfo_size doesn't match the space used by
rtnl_fill_vfinfo.

rtnl_vfinfo_size currently doesn't account for the nest attributes
used by statistics (added in commit 3b766cd8), nor for struct
ifla_vf_tx_rate (since commit ed616689, which added ifla_vf_rate
to the dump without removing ifla_vf_tx_rate, but replaced
ifla_vf_tx_rate with ifla_vf_rate in the size computation).

Fixes: 3b766cd8 ("net/core: Add reading VF statistics through the PF netdevice")
Fixes: ed616689

 ("net-next:v4: Add support to configure SR-IOV VF minimum and maximum Tx rate through ip tool")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>