flow-accounting: T3132: fix egress iptables chain

[jpbede]

Change Summary

Use the right iptables chain for egress flow sampling.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://phabricator.vyos.net/T3132

Component(s) name

flow-accounting

Proposed changes

Use the right iptables chain for egress flow sampling. Used POSTROUTING, which indeed includes packets created by local processes. Those packets shouldn't be sampled as also done by ingress flow sampling.

This fix also should be cherry-picked in to the current branch.

Please see the comments on the linked task for the problem.

How to test

I've tested it on my lab router. After i applied the proposed change, the samples with macaddress 00:00:00:00:00:00 displayed by show flow-accounting disappeared. Same i can confirm with my sFlow collector.

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly