scsi: zero per-cmd driver data before each I/O

Without this drivers that don't clear the state themselves can see off
effects.  For example Hyper-V VMs using the storvsc driver will often
hang during boot due to uncleared Test Unit Ready failures.

Fixes: e9c787e6 ("scsi: allocate scsi_cmnd structures as part of struct request")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Dexuan Cui <decui@microsoft.com>
Tested-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Jens Axboe <axboe@fb.com>