gpt-auto-generator: Use volatile-root by default and automatic logic as fallback

gpt-auto-generator: Use volatile-root by default and automatic logic as fallback

Previously volatile-root was only checked if "/" wasn't backed by a
block device, but the block device isn't necessarily original root block
device (ex: if the rootfs is copied to a ext4 fs backed by zram in the
initramfs), so we always want volatile-root checked.

So shuffle the code around so volatile-root is checked first and
fallback to the automatic logic.

Fix #20557