packetbeat/decoder: reset multilayer decoding layers for each sniffed packet

[efd6]

The current logic for multilayer use means that sucessive packets in the stream from the sniffer can end up as parts of the multilayer although they are not related layers. The documentation for the type says it is provided to allow "switching between multiple layers to remember outer layer results", but successive packet in the sniffer stream do not have this relationship.

For #33012

[mergify[bot]]

This pull request does not have a backport label. If this is a bug or security fix, could you label this PR @efd6? 🙏. For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change) To fixup this pull request, you need to add the backport labels for the needed branches, such as:
• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

:green_heart: Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-09-20T23:29:30.936+0000

• Duration: 44 min 46 sec

Test stats :test_tube:

Test	Results
Failed	0
Passed	1709
Skipped	19
Total	1728

:green_heart: Flaky test report

Tests succeeded.

:robot: GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[efd6]

I don't know why the cnt field was added; I have been using it as a sequence number while debugging, and I suspect that was the reason it was originally included. In the long run, I think it can probably go away, but it's useful at the moment.