drm/shmem-helpers: Ensure get_pages is not called on imported dma-buf

Just a bit of light paranoia. Also sprinkle this check over
drm_gem_shmem_get_sg_table, which should only be called when
exporting, same for the pin/unpin functions, on which it relies to
work correctly.

Acked-by: Thomas Zimmermann <[e-mail]>
Tested-by: Boris Brezillon <[e-mail]>
Cc: Gerd Hoffmann <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: Noralf Trønnes <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem_shmem_helper.c

···
 {
 	int ret;
 
+	WARN_ON(shmem->base.import_attach);
+
 	ret = mutex_lock_interruptible(&shmem->pages_lock);
 	if (ret)
 		return ret;
···
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 
+	WARN_ON(shmem->base.import_attach);
+
 	return drm_gem_shmem_get_pages(shmem);
 }
 EXPORT_SYMBOL(drm_gem_shmem_pin);
···
 void drm_gem_shmem_unpin(struct drm_gem_object *obj)
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
+
+	WARN_ON(shmem->base.import_attach);
 
 	drm_gem_shmem_put_pages(shmem);
 }
···
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 	int ret;
 
+	WARN_ON(shmem->base.import_attach);
+
 	ret = drm_gem_shmem_get_pages(shmem);
 	WARN_ON_ONCE(ret != 0);
 
···
 struct sg_table *drm_gem_shmem_get_sg_table(struct drm_gem_object *obj)
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
+
+	WARN_ON(shmem->base.import_attach);
 
 	return drm_prime_pages_to_sg(shmem->pages, obj->size >> PAGE_SHIFT);
 }