Giant held. In camisr(), move the ccb_bioq elements to a temporary local list
and then process the elements off of that list. This enables the list to be
processed by only taking the ccb_bioq_lock once and only for a very short
time.
ccb_bioq_lock is a leaf mutex, so it's fine to call xpt_done() with other
locks held. This is just a very minor step in the work to lock CAM, but
it allows us to avoid some messy locking/unlock dances in certain drivers.
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