nixos/filesystems: Improve vboxsf default options.

The default options for all file systems currently are
"defaults.relatime", which works well on file systems which support the
relatime option.

Unfortunately, this is not the case for the VirtualBox shared folder
filesystem, so until now, you need to set something like:

fileSystems."/foo" = {
  device = "foo";
  fsType = "vboxsf";
  options = "defaults";
};

Otherwise mounting the file system would fail.

Now, we provide only the "defaults" option to the "vboxsf" file system,
so something like this is enough:

fileSystems."/foo" = {
  device = "foo";
  fsType = "vboxsf";
};

An alternative to that could be to document that you need to set default
options, but we really should do what users expect instead of forcing
them to look up the documentation as to why this has failed.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/tasks/filesystems.nix

@@ -69,6 +69,8 @@ let
     config = {
       mountPoint = mkDefault name;
       device = mkIf (config.fsType == "tmpfs") (mkDefault config.fsType);
+      # The vboxsf filesystem doesn't support the relatime option:
+      options = mkIf (config.fsType == "vboxsf") (mkDefault "defaults");
     };
 
   };