eth: fbnic: use the Rx queue napi pointer to find the napi vector

In the Linux kernel, the following vulnerability has been resolved:

eth: fbnic: use the Rx queue napi pointer to find the napi vector

The queue management ndos pick the napi vector for an Rx queue with:
nv = fbn->napi[idx % fbn->num_napi];

The issue is this is only correct in the cases where there are no
standalone Tx vectors. In those cases we were allocating the Tx vectors
first and then the Rx so the queues would be pointing to Tx NAPI vectors
instead of the Rx ones.

The mapping the ndos want is already recorded. fbnic_set_netif_napi()
publishes it with netif_queue_set_napi(), which stores the napi pointer
in netdev_rx_queue.napi, and fbnic_reset_netif_napi() clears it again.
Both run under the netdev instance lock that the queue management ndos
also hold, so the pointer can be read directly.

Use it and drop the divide. The pointer is NULL exactly while the
datapath is down, so fbnic_queue_mem_alloc() can reject that case rather
than reaching into freed state: netdev_rx_queue_restart() calls it
before it tests netif_running(), and fbnic_pm_suspend() leaves
netif_running() true across a PCIe recovery that never completes, so a
queue restart can arrive after fbnic_stop() has freed the rings and the
vectors. fbnic_stop() clears the association in
fbnic_reset_netif_queues() before fbnic_free_napi_vectors(), so the
NULL is always published first. fbnic_queue_start() and
fbnic_queue_stop() need no check of their own, as
netdev_rx_queue_reconfig() only reaches them once fbnic_queue_mem_alloc()
has succeeded under the same instance lock.