*Scan: Scan only usable part of single-slot slot-spans

This makes sure to avoid scanning/faulting pages that are located after
the end of a large allocation.

Bug: 11297512
Change-Id: I062ad44229c5dc5c8f768221b1a18299cc0f92fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3009822


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#899146}