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: 2ceda20f ("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>