Cherry-pick #19131 to 7.8: [filebeat][netflow] Support up to 7 bytepad for Netflow IPFIX template

[marc-gr]

Cherry-pick of PR #19131 to 7.8 branch. Original message:

What does this PR do?

Fixes netflow v9 decoder to support up to 7 bytes of padding for IPFIX templates.

Why is it important?

Previously was failing to process packets with more than 3 bytes of padding.

Checklist

• My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

Fixes #18098

[botelastic]

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[elasticmachine]

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Build stats

• Build Cause: [Pull request #19190 opened]

• Start Time: 2020-06-15T10:13:08.709+0000

• Duration: 45 min 49 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1686
Skipped	255
Total	1941