gen_stats: Move remaining users to gnet_stats_add_queue().

The gnet_stats_queue::qlen member is only used in the SMP-case.

qdisc_qstats_qlen_backlog() needs to add qdisc_qlen() to qstats.qlen to
have the same value as that provided by qdisc_qlen_sum().

gnet_stats_copy_queue() needs to overwritte the resulting qstats.qlen
field whith the caller submitted qlen value. It might be differ from the
submitted value.

Let both functions use gnet_stats_add_queue() and remove unused
__gnet_stats_copy_queue().

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