Validate the tx buffer should be enough to gurantee pkts not drop at tx side

[kevinskwang]

Description

All the admission control should be done at the ingress side. Tx side should guarantee those pkts could be all sent out. Steps to reproduce the issue: 1. 2. 3.

Describe the results you received:

Describe the results you expected:

Additional information you deem important:

**Output of `show version`:**

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**Attach debug file `sudo generate_dump`:**

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[lguohan]

this should be applied to lossless traffic only, for lossy traffic tx drop is expected since there is no ingress admission control. but for lossless the admission control is at the ingress, there should be no admission control for the egress.