f2fs: remove build_free_nids() during checkpoint

Let's avoid build_free_nids() in checkpoint path.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -999,8 +999,6 @@ static int block_operations(struct f2fs_sb_info *sbi)
 static void unblock_operations(struct f2fs_sb_info *sbi)
 {
 	up_write(&sbi->node_write);
-
-	build_free_nids(sbi, false);
 	f2fs_unlock_all(sbi);
 }