Decouple bsg from scsi_cmd_ioctl().  This requires a small amount of code
duplication, but will allow moving all SCSI ioctl handling into SCSI
midlayer.

Link: https://lore.kernel.org/r/20210724072033.1284840-14-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>