scsi: core: Rename scsi_mq_prep_fn() to scsi_prepare_cmd()

The old name is rather confusing now that the the legacy prep_fn is gone.

Link: https://lore.kernel.org/r/20201005084130.143273-8-hch@lst.de


Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>