Make aof buf alloc size more accurate in overhead (#8970)

In theory we should have used zmalloc_usable_size, but it may be slower,
and now after #7875, there's no actual difference.
So this change isn't making a dramatic change.

Co-authored-by: Oran Agra <oran@redislabs.com>

src/evict.c

@@ -342,7 +342,7 @@ size_t freeMemoryGetNotCountedMemory(void) {
         }
     }
     if (server.aof_state != AOF_OFF) {
-        overhead += sdsalloc(server.aof_buf)+aofRewriteBufferSize();
+        overhead += sdsAllocSize(server.aof_buf)+aofRewriteBufferSize();
     }
     return overhead;
 }