scsi: core: Only start the request just before dispatching

This has no change in behavior, but improves the accounting a bit.

Link: https://lore.kernel.org/r/20201005084130.143273-11-hch@lst.de


Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>