f2fs: avoid infinite loop in f2fs_alloc_nid

If we have an error in f2fs_build_free_nids, we're able to fall into a loop
to find free nids.

Suggested-by: Chao Yu <chao@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>