When link-layer information is wrapped with sockaddr_dl, we need to
follow the len field of sockaddr_dl. When link-layer information is
naked, we need to use the length of whole link-layer information.

Fixes #12641.

Change-Id: I4d377f64cbab1760b993fc55c719288616042bbb
Reviewed-on: https://go-review.googlesource.com/14939Reviewed-by: Ian Lance Taylor <iant@golang.org>