lib/repo: Immediately error creating bare-user repo on tmpfs

And in general, if for some reason we can't write `user.` xattrs, provide an
error immediately rather than doing it during a later pull. This way the failure
cause is a lot more obvious.

Related: https://github.com/ostreedev/ostree/issues/991

Closes: #993
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -216,6 +216,14 @@ _ostree_repo_has_loose_object (OstreeRepo           *self,
                                GCancellable         *cancellable,
                                GError             **error);
 
+gboolean
+_ostree_write_bareuser_metadata (int fd,
+                                 guint32       uid,
+                                 guint32       gid,
+                                 guint32       mode,
+                                 GVariant     *xattrs,
+                                 GError       **error);
+
 gboolean
 _ostree_repo_write_directory_meta (OstreeRepo   *self,
                                    GFileInfo    *file_info,