drm/shmem-helper: Remove another errant put in error path

commit ee9adb7a45 upstream.

drm_gem_shmem_mmap() doesn't own reference in error code path, resulting
in the dma-buf shmem GEM object getting prematurely freed leading to a
later use-after-free.

Fixes: f49a51bfdc ("drm/shme-helpers: Fix dma_buf_mmap forwarding bug")
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230108211311.3950107-1-dmitry.osipenko@collabora.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -591,11 +591,14 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct
 	int ret;
 
 	if (obj->import_attach) {
-		/* Drop the reference drm_gem_mmap_obj() acquired.*/
-		drm_gem_object_put(obj);
 		vma->vm_private_data = NULL;
+		ret = dma_buf_mmap(obj->dma_buf, vma, 0);
 
-		return dma_buf_mmap(obj->dma_buf, vma, 0);
+		/* Drop the reference drm_gem_mmap_obj() acquired.*/
+		if (!ret)
+			drm_gem_object_put(obj);
+
+		return ret;
 	}
 
 	ret = drm_gem_shmem_get_pages(shmem);