nixos/filesystems: escape mount options in fstab

Some mount options might include path names and those often contain spaces and
therefore must be escaped. An example which prompted me to make this change is
the path of a btrfs subvolume.

nixos/modules/tasks/filesystems.nix

@@ -167,7 +167,7 @@ let
          else throw "No device specified for mount point ‘${fs.mountPoint}’.")
       + " " + escape (rootPrefix + fs.mountPoint)
       + " " + fs.fsType
-      + " " + builtins.concatStringsSep "," (fs.options ++ (extraOpts fs))
+      + " " + escape (builtins.concatStringsSep "," (fs.options ++ (extraOpts fs)))
       + " " + (optionalString (!excludeChecks)
         ("0 " + (if skipCheck fs then "0" else if fs.mountPoint == "/" then "1" else "2")))
       + "\n"