[scudo] Fix a bug in the new Secondary Allocator

Summary:
GetActuallyAllocatedSize() was not accounting for the last page of the mapping
being a guard page, and was returning the wrong number of actually allocated
bytes, which in turn would mess up with the realloc logic. Current tests didn't
find this as the size exercised was only serviced by the Primary.

Correct the issue by subtracting PageSize, and update the realloc test to
exercise paths in both the Primary and the Secondary.

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24787

llvm-svn: 282030