kselftests: dmabuf-heaps: Ensure the driver name is null-terminated

Even if a vgem device is configured in, we will skip the import_vgem_fd()
test almost every time.

  TAP version 13
  1..11
  # Testing heap: system
  # =======================================
  # Testing allocation and importing:
  ok 1 # SKIP Could not open vgem -1

The problem is that we use the DRM_IOCTL_VERSION ioctl to query the driver
version information but leave the name field a non-null-terminated string.
Terminate it properly to actually test against the vgem device.

While at it, let's check the length of the driver name is exactly 4 bytes
and return early otherwise (in case there is a name like "vgemfoo" that
gets converted to "vgem\0" unexpectedly).
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240729024604.2046-1-yuzenghui@huawei.com

tools/testing/selftests/dmabuf-heaps/dmabuf-heap.c

@@ -29,9 +29,11 @@ static int check_vgem(int fd)
 	version.name = name;
 
 	ret = ioctl(fd, DRM_IOCTL_VERSION, &version);
-	if (ret)
+	if (ret || version.name_len != 4)
 		return 0;
 
+	name[4] = '\0';
+
 	return !strcmp(name, "vgem");
 }