Major Description
When we started routers with dfs.federation.router.quota.enable=false, and try to setQuota through them, NullPointerException caught.
The cuase of NPE is that the Router#quotaManager not initialized when dfs.federation.router.quota.enable=false,
but when executing setQuota rpc request inside router, we wolud use it in method Quota#isMountEntry without null check .
I think it's better to check whether Router#isQuotaEnabled is true before use Router#quotaManager, and throw an IOException with readable message if need.