8231774: ZGC: ZVirtualMemoryManager unmaps incorrect address

Reviewed-by: kbarrett, stefank

src/hotspot/os/posix/gc/z/zVirtualMemory_posix.cpp

@@ -43,7 +43,7 @@ static bool map(uintptr_t start, size_t size) {
 
   if ((uintptr_t)res != start) {
     // Failed to reserve memory at the requested address
-    unmap(start, size);
+    unmap((uintptr_t)res, size);
     return false;
   }