sch_tbf: use do_div() for 64-bit divide

It's doing a 64-bit divide which is not supported
on 32-bit architectures in psched_ns_t2l(). The
correct way to do this is to use do_div().

It's introduced by commit cc106e44
("net: sched: tbf: fix the calculation of max_size")

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>