fsck.f2fs: reconnect unreachable files to lost+found

fsck.f2fs: reconnect unreachable files to lost+found

This patch introduces lost_found feature to fsck. If a file is found
unreachable by fsck. Fsck tries to reconnect the file to lost+found
directory:
  1. Scan all unreachable file inodes, ignore non-inodes ones and
     directories.
  2. Check them and fix incorrupted data to make sure filesystem
     metadata (mainly counters and main/nat bitmap) are all consistent.
  3. Reconnect these files to lost+found. If lost+found does not exist,
     create it first. During reconnecting, expand lost+found's dentry
     block automatically. Reconnected files are renamed after its ino
     number.
  4. If reconnect fails drop the node and restore filesystem metadata.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>