T5872: fix ipsec dhclient hook uses "exit" instead of "return" (backport #3302)

[mergify]

Change Summary

This is a bug fix to #2965.
Because dhclient sources each exit hook instead of running it in a separate shell, we need to use return to bail out early, not exit.

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):

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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

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This is an automatic backport of pull request #3302 done by [Mergify](https://mergify.com).