fix(dracut-init.sh): instmods: wrong variable name

It worked most of the time because `dracut-install -o` returns 0 in normal circumstances.

Author: marcosfrm

dracut-init.sh

@@ -1138,7 +1138,7 @@ instmods() {
             -m "$@"
     fi
 
-    [[ "$optional" ]] && return 0
+    [[ "$_optional" ]] && return 0
     return $_ret
 }