userns: don't leak root user

Signed-off-by: Alexey Dobriyan <[e-mail]>
Acked-by: Cedric Le Goater <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/user_namespace.c

···
 	struct user_namespace *ns;
 
 	ns = container_of(kref, struct user_namespace, kref);
+	free_uid(ns->root_user);
 	kfree(ns);
 }