[FS#597] Unchecking "Use DNS servers advertised by peer" has no effect for IPv6

[LEDE Bugs]

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FS#597 - Unchecking "Use DNS servers advertised by peer" has no effect for IPv6
User who did this: Jo-Philipp Wich (jow-)
Status: Unconfirmed -> Requires testing

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=597