ixgbe: Fix NULL pointer dereference in ethtool loopback test

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.

Fixes: 3e7eb5f5334a ("xsk: Propagate napi_id to XDP socket Rx path")
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>