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

[LEDE Bugs]

The following task has a new comment added:

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

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I staged a proposed fix at https://git.lede-project.org/0cd002a and asked dedeckeh to help testing it.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=597#comment2077