drm: drm_ioctl() should zero-init extra data

If an older userspace passes in a smaller arg than the current kernel
ioctl arg struct, then extra fields should be initialized to zero
rather than passing random data to the DRM driver.
Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_drv.c

@@ -438,6 +438,8 @@ long drm_ioctl(struct file *filp,
 					goto err_i1;
 				}
 			}
+			if (asize > usize)
+				memset(kdata + usize, 0, asize - usize);
 		}
 
 		if (cmd & IOC_IN) {