MFC 233708,234154:

MFC 233708,234154:
Fix a few issues with transmit handling in em(4) and igb(4):
- Do not define the foo_start() methods or set if_start in the ifnet if
  multiq transmit is enabled.  Also, set if_transmit and if_qflush before
  ether_ifattach rather than after when multiq transmit is enabled.  This
  helps to ensure that the drivers never try to mix different transmit
  methods.
- Properly restart transmit during resume.  igb(4) was not restarting it
  at all, and em(4) was restarting even if the link was down and was
  calling the wrong method if multiq transmit was enabled.
- Remove all the 'more' handling for transmit completions.  Transmit
  completion processing does not have a processing limit, so it always
  runs to completion and never has more work to do when it returns.
  Instead, the previous code was returning 'true' anytime there were
  packets in the queue that weren't still in the process of being
  transmitted.  The effect was that the driver would continuously
  reschedule a task to process TX completions in effect running at 100%
  CPU polling the hardware until it finished transmitting all of the
  packets in the ring.  Now it will just wait for the next TX completion
  interrupt.
- Restart packet transmission when the link becomes active.
- Fix the MSI-X queue interrupt handlers to restart packet transmission if
  there are pending packets in the relevant software queue (IFQ or buf_ring)
  after processing TX completions.  This is the root cause for the OACTIVE
  hangs as if the MSI-X queue handler drained all the pending packets from
  the TX ring, nothing would ever restart it.  As such, remove some
  previously-added workarounds to reschedule a task to poll the TX ring
  anytime OACTIVE was set.
- 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.
- 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@235553 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f