CVE-2021-46916 ixgbe: Fix NULL pointer dereference in ethtool loopback test

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

ixgbe: Fix NULL pointer dereference in ethtool loopback test

The ixgbe driver currently generates a NULL pointer dereference when
performing the ethtool loopback test. This is due to the fact that there
isn’t a q_vector associated with the test ring when it is setup as
interrupts are not normally added to the test rings.

To address this I have added code that will check for a q_vector before
returning a napi_id value. If a q_vector is not present it will return a
value of 0.