veth: Enforce minimum size of VETH_INFO_PEER

VETH_INFO_PEER carries struct ifinfomsg plus optional IFLA
attributes. A minimal size of sizeof(struct ifinfomsg) must be
enforced or we may risk accessing that struct beyond the limits
of the netlink message.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>