net: ifb: support ethtools stats

With this feature, we can use the ethtools to get tx/rx
queues stats. This patch, introduce the ifb_update_q_stats
helper to update the queues stats, and ifb_q_stats to simplify
the codes.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Link: https://lore.kernel.org/r/20211128014631.43627-1-xiangxia.m.yue@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>