MFC 223198:

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().

git-svn-id: svn://svn.freebsd.org/base/stable/8@223675 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f