[scudo] Use bytes-in-freelist as a hint of page release

Tracking the pushed bytes between to releaseToOSMaybe calls may lead to
a overestimated case that if we do malloc 2KB -> free 2KB -> malloc 2KB
-> free 2KB, we may think we have released 4KB but it only releases 2KB
actually. Switch to use bytes-in-freelist excludes more cases that can't
release the pages

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D146400