eth: fbnic: reset num_napi when the napi vectors are freed

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

eth: fbnic: reset num_napi when the napi vectors are freed

fbn->num_napi is the count of live napi vectors, each of which owns an
IRQ. The PM path had freed them without clearing the count.
fbnic_pm_suspend() tears the datapath down via ndo_stop() and frees the
IRQs, but leaves netif_running() true so resume knows to re-open. Resume
rebuilds the datapath in __fbnic_pm_resume() and fbnic_reset_queues() sets
num_napi and __fbnic_open() re-allocates the vectors.

When the datapath is torn down but never rebuilt, num_napi is left
pointing at freed vectors under 2 different scenarios:
- a PCIe error recovery that fails (fbnic_err_slot_reset() ->
__fbnic_pm_resume() returns an error -> PCI_ERS_RESULT_DISCONNECT), so
.resume never runs; or
- an __fbnic_open() that fails partway on resume and unwinds, freeing
the vectors after fbnic_reset_queues() has already set num_napi.

The netdev is then running with num_napi > 0 but napi[] freed, and the
eventual remove/unbind close re-enters fbnic_down() -> fbnic_dbg_down()
and dereferences the freed vectors:
BUG: kernel NULL pointer dereference, address: 0000000000000210
RIP: fbnic_dbg_down+0x28

Clear num_napi when the vectors are freed: in the suspend teardown (a
good resume re-establishes it before __fbnic_open()) and on the resume
open failure. A redundant ndo_stop() then walks an empty napi[]. The
normal ndo_stop() down/up cycle is untouched and keeps num_napi for the
next ndo_open().