drm/msm: Don't call dma_buf_vunmap without _vmap

I honestly don't exactly understand what's going on here, but the
current code is wrong for sure: It calls dma_buf_vunmap without ever
calling dma_buf_vmap.

What I'm not sure about is whether the WARN_ON is correct:
- msm imports dma-buf using drm_prime_sg_to_page_addr_arrays. Which is
  a pretty neat layering violation of how you shouldn't peek behind
  the curtain of the dma-buf exporter, but par for course. Note that
  all the nice new helpers don't (and we should probably have a bit a
  warning about this in the kerneldoc).

- but then in the get_vaddr() in msm_gem.c, we seems to happily wrap a
  vmap() around any object with ->pages set (so including imported
  dma-buf).

- I'm not seeing any guarantees that userspace can't use an imported
  dma-buf for e.g. MSM_SUBMIT_CMD_BUF in a5xx_submit_in_rb, so no
  guarantees that an imported dma-buf won't end up with a ->vaddr set.

But even if that WARN_ON is wrong, cleaning up a vmap() done by msm by
calling dma_buf_vunmap is the wrong thing to do.

v2: Rob said in review that we do indeed have a gap in get_vaddr() that
needs to be plugged. But the users I've found aren't legit users on
imported dma-buf, so we can just reject that.
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200514201117.465146-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/msm/msm_gem.c

@@ -554,6 +554,9 @@ static void *get_vaddr(struct drm_gem_object *obj, unsigned madv)
 	struct msm_gem_object *msm_obj = to_msm_bo(obj);
 	int ret = 0;
 
+	if (obj->import_attach)
+		return ERR_PTR(-ENODEV);
+
 	mutex_lock(&msm_obj->lock);
 
 	if (WARN_ON(msm_obj->madv > madv)) {
@@ -907,8 +910,7 @@ static void free_object(struct msm_gem_object *msm_obj)
 	put_iova(obj);
 
 	if (obj->import_attach) {
-		if (msm_obj->vaddr)
-			dma_buf_vunmap(obj->import_attach->dmabuf, msm_obj->vaddr);
+		WARN_ON(msm_obj->vaddr);
 
 		/* Don't drop the pages for imported dmabuf, as they are not
 		 * ours, just free the array we allocated: