fs: don't forget to put old mntns in mntns_install

Fixes: 4f757f3cbf54 ("make sure that mntns_install() doesn't end up with referral for root")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -3488,6 +3488,8 @@ static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 		return err;
 	}
 
+	put_mnt_ns(old_mnt_ns);
+
 	/* Update the pwd and root */
 	set_fs_pwd(fs, &root);
 	set_fs_root(fs, &root);