dracut.sh: Fix variable name typo.

This caused the root_dev variable not to be set which in turn meant that
the root device was not whitelisted in 99base/module-setup.sh when injecting
compile-time devexists hooks in hostonly initrds. This ties the generated
initrd to the root fs device (typically the UUID) rather than relying solely
only the root= kernel command line.

While it is hostonly, not hardcoding e.g. UUIDs is still desirable. Any
swap partition on the host device is still added however.