[scudo] Extract steps releaseToOSMaybe into functions in

This refactor helps us identify which steps need FLLock so that we can
reduce the holding time of FLLock in SizeClassAllocator64.

Also move the data members to the end of class to align the style in
SizeClassAllocator32.

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D152596