f2fs: skip releasing nodes in chindless extent tree

If there are no nodes in extent tree, let's skip releasing step to avoid
any overhead of grabbing/releasing extent tree lock.
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/extent_cache.c

@@ -570,9 +570,11 @@ unsigned int f2fs_shrink_extent_tree(struct f2fs_sb_info *sbi, int nr_shrink)
 
 	/* 1. remove unreferenced extent tree */
 	list_for_each_entry_safe(et, next, &sbi->zombie_list, list) {
+		if (atomic_read(&et->node_cnt)) {
 			write_lock(&et->lock);
 			node_cnt += __free_extent_tree(sbi, et, true);
 			write_unlock(&et->lock);
+		}
 
 		list_del_init(&et->list);
 		radix_tree_delete(&sbi->extent_tree_root, et->ino);
@@ -618,6 +620,9 @@ unsigned int f2fs_shrink_extent_tree(struct f2fs_sb_info *sbi, int nr_shrink)
 		for (i = 0; i < found; i++) {
 			struct extent_tree *et = treevec[i];
 
+			if (!atomic_read(&et->node_cnt))
+				continue;
+
 			if (write_trylock(&et->lock)) {
 				node_cnt += __free_extent_tree(sbi, et, false);
 				write_unlock(&et->lock);
@@ -641,7 +646,7 @@ unsigned int f2fs_destroy_extent_node(struct inode *inode)
 	struct extent_tree *et = F2FS_I(inode)->extent_tree;
 	unsigned int node_cnt = 0;
 
-	if (!et)
+	if (!et || !atomic_read(&et->node_cnt))
 		return 0;
 
 	write_lock(&et->lock);