scsi: sr: Initialize ->cmd_len

Ensure the command length is properly set.  Previously the command code
tried to find this out using the command opcode.

Link: https://lore.kernel.org/r/20201008200611.1818099-2-hch@lst.de
Fixes: 2ceda20 ("scsi: core: Move command size detection out of the fast path")
Reported-by: Qian Cai <cai@redhat.com>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: Christoph Hellwig

drivers/scsi/sr.c

@@ -503,6 +503,7 @@ static blk_status_t sr_init_command(struct scsi_cmnd *SCpnt)
 	SCpnt->transfersize = cd->device->sector_size;
 	SCpnt->underflow = this_count << 9;
 	SCpnt->allowed = MAX_RETRIES;
+	SCpnt->cmd_len = 10;
 
 	/*
 	 * This indicates that the command is ready from our end to be queued.