CL 496037 had ignored 3 types of null route, however blackhole route
is not included i.e. on Linux we can add a blackhole route by
`ip route add blackhole 198.18.0.254/32`
Fixes #61590
Change-Id: I9ddb86c5be0e5e261afa96cbaf55d9fdf30b2795
Reviewed-on: https://go-review.googlesource.com/c/go/+/513595
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