fsck.ubifs: Check and handle invalid files

fsck.ubifs: Check and handle invalid files

This is the 8/18 step of fsck. Check and handle invalid files, the
checking rule is same as rebuild mode which has been implemented in
file_is_valid, but the methods of handling are different:
 1. Move unattached(file has no dentries) regular file into disconnected
    list, let subsequent steps to handle them with lost+found.
 2. Make file type be consistent between inode, detries and data nodes by
    deleting dentries or data blocks.
 3. Delete file for other invalid cases(eg. file has no inode).

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