ftl: change order of decrementing the used page count to be more logical

source/ftl/ftl.c

@@ -175,8 +175,6 @@ int ftl_trim(struct ftl_map *map, uint32_t va)
 		return ret;
 	}
 
-	--map->nused_pages;
-
 	for (i = 0; i < 32; ++i) {
 		level = 32 - i - 1;
 
@@ -186,6 +184,7 @@ int ftl_trim(struct ftl_map *map, uint32_t va)
 
 	if (i == 32) {
 		map->root = UINT32_MAX;
+		map->nused_pages = 0;
 
 		memset(&page_desc, 0xff, sizeof page_desc);
 		page_desc.nused_pages = 0;
@@ -196,6 +195,8 @@ int ftl_trim(struct ftl_map *map, uint32_t va)
 	if (read_page_desc(map, &alt_page_desc, alt_page) < 0)
 		return -1;
 
+	--map->nused_pages;
+
 	page_desc.va = alt_page_desc.va;
 	page_desc.nused_pages = map->nused_pages;
 	page_desc.subtrees[level] = UINT32_MAX;