Removed suggestion to use root dataset as bootfs

Removed suggestion to use root dataset as bootfs

The dracut howto proposed to boot from the root dataset of a pool.
Apart from this giving problems when booting (as the code seems to
expect a child dataset and creates an illegal dataset name when using
the root dataset) the technical limitations of the root dataset
(among others the inability to rename or destroy through the `zfs`
command) resulted in the general consensus to only use it as a
container for the datasets in the pool - not as a filesystem itself.

Removed the idea to boot from the root dataset.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Signed-off-by: Gregor Kopka <gregor@kopka.net>
Closes #8247