malloc: implement malloc_usable_size().

* Fix #12132

headers/posix/malloc.h

@@ -20,6 +20,10 @@ extern void free(void *pointer);
 extern void *memalign(size_t alignment, size_t numBytes);
 extern void *valloc(size_t numBytes);
 
+#ifdef _GNU_SOURCE
+size_t malloc_usable_size(void *ptr);
+#endif
+
 #ifdef __cplusplus
 }
 #endif

src/system/libroot/posix/malloc/wrapper.cpp

@@ -490,6 +490,15 @@ realloc(void *ptr, size_t size)
 }
 
 
+extern "C" size_t
+malloc_usable_size(void *ptr)
+{
+	if (ptr == NULL)
+		return 0;
+	return threadHeap::objectSize(ptr);
+}
+
+
 //	#pragma mark - BeOS specific extensions