BUILD: pools: fix build error on DEBUG_POOL_TRACING

When squashing commit add43fa43 ("DEBUG: pools: add new build option
DEBUG_POOL_TRACING") I managed to break the build and to fail to detect
it even after the rebase and a full rebuild :-(
diff --git a/src/pool.c b/src/pool.c
index a20880b..3fda2fb 100644
--- a/src/pool.c
+++ b/src/pool.c
@@ -284,7 +284,7 @@
 
 	/* keep track of where the element was allocated from */
 	POOL_DEBUG_SET_MARK(pool, ptr);
-	POOL_DEBUG_TRACE_CALLER(pool, item, NULL);
+	POOL_DEBUG_TRACE_CALLER(pool, (struct pool_cache_item *)ptr, NULL);
 	return ptr;
 }
 
@@ -470,9 +470,9 @@
 		down = ret->down;
 		/* keep track of where the element was allocated from */
 		POOL_DEBUG_SET_MARK(pool, ret);
-		POOL_DEBUG_TRACE_CALLER(pool, item, NULL);
 
 		item = (struct pool_cache_item *)ret;
+		POOL_DEBUG_TRACE_CALLER(pool, item, NULL);
 		LIST_INSERT(&pch->list, &item->by_pool);
 		LIST_INSERT(&th_ctx->pool_lru_head, &item->by_lru);
 		count++;