Mitigate the as-yet-unknown race in device departure.
When devices are invalidated, there's some cases where ccbs for that
device still wind up in xpt_done after the path has been
invalidated. Since we don't always need sim or devq, add some guard
rails to only fail if we have to use them.
Track Ref / DeRef and Hold / Unhold that da is doing to track down
leaks. We assume each source can be taken / dropped only once and
Likely needs #ifdef INVARIANTS for upstream.