fsck.ubifs: Add fsck support

fsck.ubifs: Add fsck support

Add basic process code for fsck.ubifs. There are following modes for fsck:
 1. normal mode: Check the filesystem, ask user whether or not to fix
    the problem as long as inconsistent data is found during fs checking.
 2. safe mode: Check and safely repair the filesystem, if there are any
    data dropping operations needed by fixing, fsck will fail.
 3. danger mode: Answer 'yes' to all questions. There two sub modes:
    a) Check and repair the filesystem according to TNC, data dropping
       will be reported. If TNC/master/log is corrupted, fsck will fail.
    b) Check and forcedly repair the filesystem according to TNC, turns
       to rebuild filesystem if TNC/master/log is corrupted. Always make
       fsck succeed.
 4. check mode: Make no changes to the filesystem, only check the
    filesystem.
 5. rebuild mode: Scan entire UBI volume to find all nodes, and rebuild
    filesystem, always make fsck success.

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