scsi: return blk_status_t from scsi_init_io and ->init_command

Replace the old BLKPREP_* values with the BLK_STS_ ones that they are
converted to later anyway.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>