UPSTREAM: scsi: sg: check length passed to SG_NEXT_CMD_LEN

(cherry pick from commit bf33f87d)

The user can control the size of the next command passed along, but the
value passed to the ioctl isn't checked against the usable max command
size.

Cc: <stable@vger.kernel.org>
Signed-off-by: Peter Chang <dpf@google.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Bug: 63666227
Change-Id: I030771908aaff870e9250da8024d0e7a524d39b9