dracut/zfs-load-key.sh: properly remove prefixes

dracut/zfs-load-key.sh: properly remove prefixes

Removes the 'ZFS=' prefix from $BOOTFS instead of $root. This makes sure
that the 'zfs:' prefix remains stripped so that users with
'root=zfs:dataset' cmdline can have key loaded on boot again.

Reviewed-by: Garrett Fields <ghfields@gmail.com>
Reviewed-by: Dacian Reece-Stremtan <dacianstremtan@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Hiếu Lê <leorize+oss@disroot.org>
Closes #9520