Dracut module: fix parsing of root= kernel command-line argument

[ahesford]

Some Dracut modules may read the root= kernel command-line argument and rewrite it; in particular, rootfs-block installs a command-line hook that to canonicalize arguments that specify block devices. If the zfs module is added to an initramfs (which it is, by default, as long as the core ZFS utilities are available) on a system that does not use a ZFS root filesystem, the zfs module will overwrite the root shell variable with the contents of the kernel command-line in the process of determining if it should control mounting the root. This breaks the initramfs.

Motivation and Context

If the zfs dracut module will be automatically included in initramfs images, it should avoid corrupting root arguments when those arguments refer to non-ZFS filesystems.

This bug was identified in void-linux/void-packages#37667 and should be fixed by this patch.

Description

Simply check if the root shell variable is non-empty, which means it has already been parsed (and possibly rewritten); prefer a previously parsed version if possible. Otherwise, parse the kernel command-line directly to populate the root shell variable. Subsequent decisions about rewriting root based on the existence of ZFS indicators remain unchanged.

How Has This Been Tested?

I have no systems without ZFS roots, so this was "tested" by reasoning in a thought experiment.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[ahesford]

Deferring to #13592