This change doesn't work perfectly on IPv6-only kernels including CLAT
enabled kernels, but works enough on IPv4-only kernels.

Fixes #10721.
Updates #10729.

Change-Id: I7db0e572e252aa0a9f9f54c8e557955077b72e44
Reviewed-on: https://go-review.googlesource.com/9777Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>