Fix for zfs-dracut regression

Line 31 and 32 overwrote the ${root} variable which broke mount-zfs.sh
We have create a new variable for the dataset instead of overwriting the
${root} variable in zfs-load-key.sh${root} variable in zfs-load-key.sh

Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: Garrett Fields <ghfields@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Dacian Reece-Stremtan <dacianstremtan@gmail.com>
Closes #8913
Closes #9379