[FS#312] GRE tunnel fail to up if df option set to 0

[LEDE Bugs]

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FS#312 - GRE tunnel fail to up if df option set to 0
User who did this: Hans Dedecker (dedeckeh)
Percent Complete: 40% -> 100%

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=312