Implement realloc accounting correctly (#51027)

This was potentially making us getting wrong results.

(cherry picked from commit ce8acdd)

src/gc.c

@@ -3711,11 +3711,12 @@ JL_DLLEXPORT void *jl_gc_counted_realloc_with_old_size(void *p, size_t old, size
 
         int64_t diff = sz - old;
         if (diff < 0) {
+            diff = -diff;
             uint64_t free_acc = jl_atomic_load_relaxed(&ptls->gc_num.free_acc);
             if (free_acc + diff < 16*1024)
-                jl_atomic_store_relaxed(&ptls->gc_num.free_acc, free_acc + (-diff));
+                jl_atomic_store_relaxed(&ptls->gc_num.free_acc, free_acc + diff);
             else {
-                jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, -(free_acc + (-diff)));
+                jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, -(free_acc + diff));
                 jl_atomic_store_relaxed(&ptls->gc_num.free_acc, 0);
             }
         }
@@ -3847,11 +3848,12 @@ static void *gc_managed_realloc_(jl_ptls_t ptls, void *d, size_t sz, size_t olds
 
     int64_t diff = allocsz - oldsz;
     if (diff < 0) {
+        diff = -diff;
         uint64_t free_acc = jl_atomic_load_relaxed(&ptls->gc_num.free_acc);
         if (free_acc + diff < 16*1024)
-            jl_atomic_store_relaxed(&ptls->gc_num.free_acc, free_acc + (-diff));
+            jl_atomic_store_relaxed(&ptls->gc_num.free_acc, free_acc + diff);
         else {
-            jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, -(free_acc + (-diff)));
+            jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, -(free_acc + diff));
             jl_atomic_store_relaxed(&ptls->gc_num.free_acc, 0);
         }
     }