This change tracks the number of potential free and unscavenged huge
pages which will be used to inform the rate at which scavenging should
occur.

For #30333.

Change-Id: I47663e5ffb64cac44ffa10db158486783f707479
Reviewed-on: https://go-review.googlesource.com/c/go/+/170860
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>