net/sched: use min() macro instead of doing it manually

Fix following coccicheck warnings:
./net/sched/cls_api.c:3333:17-18: WARNING opportunity for min()
./net/sched/cls_api.c:3389:17-18: WARNING opportunity for min()
./net/sched/cls_api.c:3427:17-18: WARNING opportunity for min()

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>