kernel - Fix another AHCI bug

kernel - Fix another AHCI bug

* Remove the unlock/lock sequences around the xpt_done() calls.  These
  temporary unlocks create a gap which can allow another interrupt to
  squeeze in and interfere with the interrupt thread that is already
  running, resulting in corruption.

  This bug occurs under very heavy loads, and typically required multiple
  concurrent ops to a SSD to trigger.

* Add additional assertions to catch issues and reorder one of the
  chiploads.

* This is a bit non-optimal, be on the lookout for deadlocks in case it
  turns out that holding the lock is a bad idea.