net: sched: move and reuse mq_change_real_num_tx()

The code for handling active queue changes is identical
between mq and mqprio, reuse it.

Suggested-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>