nfs mount over VPN tunnel does not work

[Wang Xuancong]

nfs mount over VPN tunnel does not work while other nfs mount works.
I have physical network interface eth0 on 192.168.101.1, and OpenVPN tunnel tun0 on 20.8.0.1.

If I try to nfs mount over VPN, I get the following error:
```
root@wxc-dell:/# mount -v 20.8.0.1:/netboot/nfs /mnt/a
mount.nfs: timeout set for Fri Apr 19 13:27:51 2024
mount.nfs: trying text-based options 'vers=4.2,addr=20.8.0.1,clientaddr=20.8.0.1'
mount.nfs: mount(2): No such file or directory
mount.nfs: trying text-based options 'addr=20.8.0.1'
mount.nfs: prog 100003, trying vers=3, prot=6
mount.nfs: trying 20.8.0.1 prog 100003 vers 3 prot TCP port 2049
mount.nfs: prog 100005, trying vers=3, prot=17
mount.nfs: trying 20.8.0.1 prog 100005 vers 3 prot UDP port 49387
mount.nfs: mount(2): Permission denied
mount.nfs: mounting 20.8.0.1:/netboot/nfs failed, reason given by server: No such file or directory
```

If I downgrade nfs-kernel-server from 2.6.1-1ubuntu1.2 to Debian Bullseye's version (1.3.4-6), together with dependencies, then everything works.

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: nfs-kernel-server 1:2.6.1-1ubuntu1.2
ProcVersionSignature: Ubuntu 6.5.0-27.28~22.04.1-generic 6.5.13
Uname: Linux 6.5.0-27-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
.etc.request-key.d.id_resolver.conf: create id_resolver * * /usr/sbin/nfsidmap -t 600 %k %d
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: KDE
Date: Fri Apr 19 13:23:40 2024
InstallationDate: Installed on 2024-04-08 (10 days ago)
InstallationMedia: Ubuntu 22.04.4 LTS "Jammy Jellyfish" - Release amd64 (20240220)
NFSMounts:

NFSv4Mounts:

SourcePackage: nfs-utils
UpgradeStatus: No upgrade log present (probably fresh install)