Fix postboot command for ZFS installation.

[unbel13ver]

This fix resolves SSRCSP-5296.
Previously, zfspool has not been expanded for the system, installed on nvme drive. The issue was so that a partition's parent disk name is not resolved correctly for /dev/nvme0n1 device.

Description of changes

Checklist for things done

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• PR linked to architecture documentation and requirement(s) (ticket id)
• Test procedure described (or includes tests). Select one or more:
  • Tested on Lenovo X1 x86_64
  • Tested on Jetson Orin NX or AGX aarch64
  • Tested on Polarfire riscv64
• Author has run make-checks and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status
• Change requires full re-installation
• Change can be updated with nixos-rebuild ... switch

Instructions for Testing

Install the system and check ZFS pool size with zpool status or zpool list. It must occupy all the free space available on the disk.

• List all targets that this applies to: Lenovo-X1
• Is this a new feature
  • List the test steps to verify:
• If it is an improvement how does it impact existing functionality?

[brianmcgillion]

size of the linux filesystem increased from 14 to 952.9 GB