scsi: return correct blkprep status code in case scsi_init_io() fails.

When instrumenting the SCSI layer to run into the
!blk_rq_nr_phys_segments(rq) case the following warning emitted from the
block layer:

blk_peek_request: bad return=-22

This happens because since commit fd3fc0b4

 ("scsi: don't BUG_ON()
empty DMA transfers") we return the wrong error value from
scsi_prep_fn() back to the block layer.

[mkp: silenced checkpatch]

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Fixes: fd3fc0b4

 scsi: don't BUG_ON() empty DMA transfers
Cc: <stable@vger.kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>