scsi: always zero sshdr in scsi_normalize_sense

This gives us a clear state even if a command didn't return sense data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>