fstab handling fails when using ZFS as root partition

[NathanaelA]

First this isn't a issue with ZFS directly, it is because of some very BAD assumptions that pop-upgrade makes.
It is an issue with pop-upgrade attempting to find a "/" mount point, if you don't have one in your fstab, it dies with:

pop-upgrade[73269]: pop-upgrade set on hold.
pop-upgrade[73268]: mount: /: can't find in /etc/fstab.
pop-upgrade[73269]: Canceled hold on pop-upgrade.

If you attempt to work around this by creating a fstab entry point for "/" pointing to the zfs partion you get:

pop-upgrade[81298]: pop-upgrade set on hold.
pop-upgrade[81308]: filesystem 'ubuntu' cannot be mounted using 'mount'.
pop-upgrade[81308]: Use 'zfs set mountpoint=legacy' or 'zfs mount ubuntu'.
pop-upgrade[81308]: See zfs(8) for more information.
pop-upgrade[81310]: Canceled hold on pop-upgrade.

Because you can't force mount -a a normal zfs partion, as zfs handles everything. (Besides that it is already mounted properly)

Related somewhat to this prior PR:

• fix: Only check if the / and /boot/efi partitions are mounted #170

Potential ways to eliminate the issue

• See if it is ZFS, and ignore the "/" requirement of it being mounted as it is already mounted.
• A optional CLI parameter to bypass fstab checks.
• Check for a "noauto" on "/" and then don't attempt to force mount it.

Work around that I used:

Added to /etc/fstab a unformated partiion and told it that it was vfat with nofail, noauto to allow reboots w/o any issues but allow the pop-upgrade to try and mount something and then to fail and ignore it. ;-)
/dev/nvme1n1 / vfat noauto,nofail 0 0