rework malloc_usable_size to use malloc_impl.h

src/malloc/malloc_usable_size.c

@@ -1,16 +1,8 @@
 #include <malloc.h>
+#include "malloc_impl.h"
 
 void *(*const __realloc_dep)(void *, size_t) = realloc;
 
-struct chunk {
-	size_t psize, csize;
-	struct chunk *next, *prev;
-};
-
-#define OVERHEAD (2*sizeof(size_t))
-#define CHUNK_SIZE(c) ((c)->csize & -2)
-#define MEM_TO_CHUNK(p) (struct chunk *)((char *)(p) - OVERHEAD)
-
 size_t malloc_usable_size(void *p)
 {
 	return p ? CHUNK_SIZE(MEM_TO_CHUNK(p)) - OVERHEAD : 0;