sch_netem: support of 64bit rates

Add a new attribute to support 64bit rates so that
tc can use them to break the 32bit limit.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>