instead of doing the first load with the BUS_DMA_NOWAIT flag. On 4.x with
PAE and > 4gb of RAM this proved disastrous if there weren't enough bounce
pages as amr_mapcmd() would return failure but the callback would later
fire once enough bounce pages were available and would then overwrite
another command's S/G list.
MFC after: 3 days
Submitted by: scottl (4.x version)
Reviewed by: scottl (port from 4.x to HEAD)
cd6451f0ca