gpu: ion: fix use-after-free in ion_heap_freelist_drain

The `buffer' variable is being used after being freed. Fix this.

Change-Id: Iea3471fa7dc7535bbf0620c1639fea2008d7cf19
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>

drivers/staging/android/ion/ion_heap.c

@@ -200,9 +200,9 @@ size_t ion_heap_freelist_drain(struct ion_heap *heap, size_t size)
 		if (total_drained >= size)
 			break;
 		list_del(&buffer->list);
-		ion_buffer_destroy(buffer);
 		heap->free_list_size -= buffer->size;
 		total_drained += buffer->size;
+		ion_buffer_destroy(buffer);
 	}
 	rt_mutex_unlock(&heap->lock);