Linux allows to have a peer IP address on IP interface over ethernet
link encapsulation, though it only installs a static route with the peer
address as an on-link nexthop.

Fixes #11338.

Change-Id: Ie2583737e4c7cec39baabb89dd732463d3f10a61
Reviewed-on: https://go-review.googlesource.com/11352Reviewed-by: Russ Cox <rsc@golang.org>