[scudo] Do not consider releasing unallocated pages

[fabio-d]

Summary

We already know that there are no free blocks above Region->AllocatedUser.
This results in a smaller RegionPageMap and faster releaseFreeMemoryToOS.

[Chia-hungDuan]

Thanks for the fix!

compiler-rt/lib/scudo/standalone/primary64.h

if (!Force) {
           getMonotonicTime()) {
         return 0; // Memory was returned recently.
       }
     }
 
     const uptr GroupSize = (1U << GroupSizeLog);
     const uptr AllocatedUserEnd = Region->AllocatedUser + Region->RegionBeg;
     ReleaseRecorder Recorder(Region->RegionBeg, &Region->Data);
-    PageReleaseContext Context(BlockSize, RegionSize, /*NumberOfRegions=*/1U);
+    PageReleaseContext Context(BlockSize, Region->AllocatedUser,
+                               /*NumberOfRegions=*/1U);
 
     const uptr CompactPtrBase = getCompactPtrBaseByClassId(ClassId);
     auto DecompactPtr = [CompactPtrBase](CompactPtrT CompactPtr) {
       return decompactPtrInternal(CompactPtrBase, CompactPtr);
     };
     for (BatchGroup &BG : Region->FreeList) {
       const uptr PushedBytesDelta =
           BG.PushedBlocks - BG.PushedBlocksAtLastCheckpoint;