mount: don't rewrite root for external mounts

It's used to restore bind-mounts. For example, we cat the common
part of bind-mounts:

Core was generated by `criu restore -vvvv --file-locks --tcp-established --evasive-devices --manage-cg'.
Program terminated with signal 11, Segmentation fault.
741                     BUG_ON(target_root[tok] == '\0');
(gdb) bt

https://jira.sw.ru/browse/PSBM-41932



Reported-by: Virtuozzo QA Team
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>