cgroup: fix missing unlock in cgroup_release_agent()

The patch 971ff493: "cgroup: use a per-cgroup work for release
agent" from Sep 18, 2014, leads to the following static checker
warning:

	kernel/cgroup.c:5310 cgroup_release_agent()
	warn: 'mutex:&cgroup_mutex' is sometimes locked here and sometimes unlocked.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NZefan Li <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -5291,7 +5291,10 @@ static void cgroup_release_agent(struct work_struct *work)
 
 	mutex_unlock(&cgroup_mutex);
 	call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
+	goto out_free;
 out:
+	mutex_unlock(&cgroup_mutex);
+out_free:
 	kfree(agentbuf);
 	kfree(pathbuf);
 }