[fstab] Do not omit "/" from crypttab.

At least the Debian update-initramfs needs the entry to be there (see
pull request #254). Dracut will probably need it too. And logically, it
should be there.

src/modules/fstab/main.py

@@ -152,9 +152,6 @@ class FstabGenerator(object):
         if not mapper_name or not luks_uuid:
             return None
 
-        if mount_point == "/":
-            return None
-
         return dict(
             name=mapper_name,
             device="UUID=" + luks_uuid,