RFC 7223, Section 3 defines 32 bits for if-index.
RFC 8335, Section 2.1 defines
"If the Interface Identification Object identifies the probed
interface by index, the length is equal to 8 and the payload contains
the if-index [RFC7223]."
The object should be comprised of a 4-byte object header and a 4-byte interface index.

Fixes golang/go#28530

Change-Id: Ib3ac729b7ec738a90a8c76ef984da0d5b28fa9c9
GitHub-Last-Rev: eba6714ed4c7af61e89f6e54d6a7544c570acebb
GitHub-Pull-Request: golang/net#23
Reviewed-on: https://go-review.googlesource.com/c/146637
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
Reviewed-by: Mikio Hara <mikioh.public.networking@gmail.com>