drm: virtio: fix kmem_cache_alloc error check

kmem_cache_alloc returns NULL on error, not ERR_PTR.

Fixes: f5985bf9cadd4e3ed8d5d9a9cbbb2e39cdb81cd9
Reported-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1489393346-13874-1-git-send-email-kraxel@redhat.com

drivers/gpu/drm/virtio/virtgpu_vq.c

@@ -97,8 +97,8 @@ virtio_gpu_get_vbuf(struct virtio_gpu_device *vgdev,
 	struct virtio_gpu_vbuffer *vbuf;
 
 	vbuf = kmem_cache_alloc(vgdev->vbufs, GFP_KERNEL);
-	if (IS_ERR(vbuf))
-		return ERR_CAST(vbuf);
+	if (!vbuf)
+		return ERR_PTR(-ENOMEM);
 	memset(vbuf, 0, VBUFFER_SIZE);
 
 	BUG_ON(size > MAX_INLINE_CMD_SIZE);