fsck.ubifs: Check and handle unreachable files

fsck.ubifs: Check and handle unreachable files

This is the 9/18 step of fsck. Check and handle unreachable files, the
checking rule is same as rebuild mode which has been implemented in
file_is_reachable, but the methods of handling are different:
1. Move unreachable regular file into disconnected list, let subsequent
   steps to handle them with lost+found.
2. Delete unreachable non-regular file.
3. Delete unreachable directory entries.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>