init: quote $rootfstype during comparison

Without this change this may result in a warning message to be printed
during boot if the rootfstype kernel parameter is not set.

initramfs-init.in

@@ -503,7 +503,7 @@ if [ -n "$KOPT_root" ]; then
 		mkdir -p /media/root-rw/work /media/root-rw/root
 		mount -t overlay -o lowerdir=/media/root-ro,upperdir=/media/root-rw/root,workdir=/media/root-rw/work overlayfs $sysroot
 	else
-		if [ $rootfstype = "zfs" ]; then
+		if [ "$rootfstype" = "zfs" ]; then
 			prepare_zfs_root
 		fi
 		mount ${rootfstype:+-t} ${rootfstype} \