Controller: Retain default backend service information.

[chessman]

What this PR does / why we need it:

This is a copy of accidentally closed PR https://github.com/kubernetes/ingress-nginx/pull/12160

When the default backend is used as upstream, the information about its service is lost. As a consequence, if the default backend is an ExternalName service, it is used as a non-ExternalName service in the Lua part.

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] CVE Report (Scanner found CVE and adding report)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation only

Which issue/s this PR fixes

fixes #12158 fixes #12173

How Has This Been Tested?

The test case is described in #12158.

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [X] I've read the CONTRIBUTION guide
• [ ] I have added unit and/or e2e tests to cover my changes.
• [X] All new and existing tests passed.