MFC 223198:

• Use a dedicated task to handle deferred transmits from the if_transmit method instead of reusing the existing per-queue interrupt task. Reusing the per-queue interrupt task could result in both an interrupt thread and the taskqueue thread trying to handle received packets on a single queue resulting in out-of-order packet processing.
• Don't define igb_start() at all on 8.0 and where if_transmit is used. Replace last remaining call to igb_start() with a loop to kick off transmit on each queue instead.
• Call ether_ifdetach() earlier in igb_detach().
• Drain tasks and free taskqueues during igb_detach().