Fix Ubuntu 24.04 shutdown hang with synced folders

[giner]

Fix Ubuntu 24.04 shutdown hang by removing _netdev from VirtualBox synced folders. Fixes #13795.

[giner]

@taru-garg-hashicorp could you help here please?

[taru-garg-2000]

Hi @giner,

Thanks for raising the PR! I am a little unsure about removing it from the mount options, wondering if this will break for people running older systems. I haven't researched it in depth, but it would be great if there is some documentation why newer versions of systemd require removal of this option.

Cheers!

[giner]

@taru-garg-hashicorp, I believe _netdev was never needed and it was added by mistake but let me try pull something and come back.

[giner]

@taru-garg-hashicorp, I didn't find out why exactly shutting down stops but I figured that this happens when /vagrant mount is mounted multiple times (once by fstab and another time by vagrant up) and _netdev option is in place. Here is another less invasive and arguably better fix #13804.

I still believe that _netdev is not needed as it makes vboxsf-based sync folders depend on the network (i.e. it won't be mounted until the network starts and the network won't stop until it's unmounted) however if another PR is merged this change would not be required to fix shutdown hanging.