net: sched: fix misuse of qcpu->backlog in gnet_stats_add_queue_cpu

In the gnet_stats_add_queue_cpu function, the qstats->qlen statistics
are incorrectly set to qcpu->backlog.

Fixes: 448e163f

 ("gen_stats: Add gnet_stats_add_queue()")
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Link: https://lore.kernel.org/r/20220815030848.276746-1-shaozhengchao@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>