scsi: Improve requeuing behavior

Requests are unprepared and reprepared when being requeued.  Avoid that
requeuing resets .jiffies_at_alloc and .retries by initializing these
two member variables from inside scsi_initialize_rq() and by preserving
both member variables when preparing a request. This patch affects the
requeuing behavior of both the legacy scsi and the scsi-mq code paths.

Reported-by: Brian King <brking@linux.vnet.ibm.com>
References: https://lkml.org/lkml/2017/8/18/923

 ("Re: [BUG][bisected 270065e] linux-next fails to boot on powerpc")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Brian King <brking@linux.vnet.ibm.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>