d_genocide() does _not_ evict dentries; it just removes extra ref
pinning each of those.  Normally it's followed by shrinking the
tree (it's done just before generic_shutdown_super() by kill_litter_super()),
but in case of simple_fill_super() nothing of that kind will follow.
Just do shrink_dcache_parent() manually.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
        return 0;
 out:
        d_genocide(root);
+       shrink_dcache_parent(root);
        dput(root);
        return -ENOMEM;
 }