files: BUG on reopening already opened fle

This point we think it's a first call of open(),
so the state must be FLE_INITIALIZED.

v6: New
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/files.c

@@ -1062,6 +1062,7 @@ static int open_fd(int pid, struct fdinfo_list_entry *fle)
 		return -1;
 	}
 
+	BUG_ON(fle->stage != FLE_INITIALIZED);
 	fle->stage = FLE_OPEN;
 
 	return serve_out_fd(pid, fle->fe->fd, d);