net: Adjust TX queue kobjects if number of queues changes during unregister

If the root qdisc for a net device is mqprio, and the driver's
ndo_setup_tc() operation dynamically adds and remvoes TX queues,
netif_set_real_num_tx_queues() will be called during device
unregistration to remove the extra TX queues when the qdisc is
destroyed.  Currently this causes the corresponding kobjects
to be leaked, and the device's reference count never drops to 0.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>