scsi: sg: check length passed to SG_NEXT_CMD_LEN

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>