mq, mqprio: Use gnet_stats_add_queue().

gnet_stats_add_basic() and gnet_stats_add_queue() add up the statistics
so they can be used directly for both the per-CPU and global case.

gnet_stats_add_queue() copies either Qdisc's per-CPU
gnet_stats_queue::qlen or the global member. The global
gnet_stats_queue::qlen isn't touched in the per-CPU case so there is no
need to consider it in the global-case.

In the per-CPU case, the sum of global gnet_stats_queue::qlen and
the per-CPU gnet_stats_queue::qlen was assigned to sch->q.qlen and
sch->qstats.qlen. Now both fields are copied individually.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>