gen_stats: Add gnet_stats_add_queue().

This function will replace __gnet_stats_copy_queue(). It reads all
arguments and adds them into the passed gnet_stats_queue argument.
In contrast to __gnet_stats_copy_queue() it also copies the qlen member.

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