Move this trivial functionality into scsi_prepare_cmd() instead of
splitting it over multiple small functions, and update the comments to
better document passthrough commands as the special case.

Link: https://lore.kernel.org/r/20201005084130.143273-10-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>