cgroup: remove synchronize_rcu() from rebind_subsystems()

Nothing's protected by RCU in rebind_subsystems(), and I can't think
of a reason why it is needed.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -1079,7 +1079,6 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 		}
 	}
 	root->subsys_mask = root->actual_subsys_mask = final_subsys_mask;
-	synchronize_rcu();
 
 	return 0;
 }