NetworkManager spams logs with IPv6 router solicitation failure for the default wired connection

Originally created by @emmapeel on #10939 (Redmine)

With Tails 2.0rc1 I get a lot of:

Jan 14 11:56:14 amnesia NetworkManager[5659]: <error> [1452772574.043792] [rdisc/nm-lndp-rdisc.c:241] send_rs(): (wlan0): cannot send router solicitation: -1.

but the network works as usual.

Feature Branch: bugfix/10939-IPv6-log-spam

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