Chain the bus_dmamap_load() calls when mapping a command with a data CCB 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.
Chain the bus_dmamap_load() calls when mapping a command with a data CCB
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)

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