Negative child dentry holds reference on inode's alias, it makes
d_prune_aliases() do nothing.

Cc: stable@vger.kernel.org
Signed-off-by: N"Yan, Zheng" <zyan@redhat.com>
Reviewed-by: NJeff Layton <jlayton@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>