Fix GRUB paths for ZFS pools/datasets

- Identify if its zfs via /proc/self/mountinfo
- get the pool/dataset (should be the 10th column)
- In the kernel line, of the /boot partition use:

    kernel /DATASET@/KERNEL_PATH
    initrd /DATASET@/INITRD_PATH

Like, if we have /boot on zroot, it should be
`/@/boot/KERNEL_PATH`, if we have it
under zroot/boot it should be `/boot@/KERNEL_PATH`, the pool
is not included

See https://groups.google.com/a/zfsonlinux.org/d/msg/zfs-discuss/jd-JoQ609pM/U9vL64sfI_IJ