[scudo] Explicit casting for u16 arithmetic operation

[Chia-hungDuan]

Summary

This fixes the werror from https://lab.llvm.org/buildbot/#/builders/165/builds/38829

compiler-rt/lib/scudo/standalone/primary32.h

for (u32 I = 0; I < Size;) {
             decompactPtr(SizeClassMap::BatchClassId, Array[I]));
         CurBatch->clear();
         // Self-contained
         CurBatch->add(Array[I]);
         ++I;
         // TODO(chiahungduan): Avoid the use of push_back() in `Batches` of
         // BatchClassId.
         BG->Batches.push_front(CurBatch);
-        UnusedSlots = BG->MaxCachedPerBatch - 1;
+        UnusedSlots = static_cast<u16>(BG->MaxCachedPerBatch - 1);
       }
       // `UnusedSlots` is u16 so the result will be also fit in u16.
       const u16 AppendSize = static_cast<u16>(Min<u32>(UnusedSlots, Size - I));
       CurBatch->appendFromArray(&Array[I], AppendSize);
       I += AppendSize;
     }
 
     BG->PushedBlocks += Size;

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

for (u32 I = 0; I < Size;) {
             decompactPtr(SizeClassMap::BatchClassId, Array[I]));
         CurBatch->clear();
         // Self-contained
         CurBatch->add(Array[I]);
         ++I;
         // TODO(chiahungduan): Avoid the use of push_back() in `Batches` of
         // BatchClassId.
         BG->Batches.push_front(CurBatch);
-        UnusedSlots = BG->MaxCachedPerBatch - 1;
+        UnusedSlots = static_cast<u16>(BG->MaxCachedPerBatch - 1);
       }
       // `UnusedSlots` is u16 so the result will be also fit in u16.
       const u16 AppendSize = static_cast<u16>(Min<u32>(UnusedSlots, Size - I));
       CurBatch->appendFromArray(&Array[I], AppendSize);
       I += AppendSize;
     }
 
     BG->PushedBlocks += Size;