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 openzfs#9520

contrib/dracut/90zfs/zfs-load-key.sh.in

@@ -30,7 +30,7 @@ if [ "${root}" = "zfs:AUTO" ] ; then
     BOOTFS="$(zpool list -H -o bootfs | awk '$1 != "-" {print; exit}')"
 else
     BOOTFS="${root##zfs:}"
-    BOOTFS="${root##ZFS=}"
+    BOOTFS="${BOOTFS##ZFS=}"
 fi
 
 # if pool encryption is active and the zfs command understands '-o encryption'