fsck.ubifs: Traverse TNC and construct files

fsck.ubifs: Traverse TNC and construct files

This is the 6/18 step of fsck. Traverse TNC and construct files. There
could be following steps and possible errors:
 Step 1. Traverse TNC, check whether the leaf node is valid, remove invalid
 nodes, construct file for valid node and insert file into file tree.
  a. corrupted node searched from TNC: remove corresponding TNC branch for
     danger mode and normal mode with 'yes' answer, other modes will exit.
  b. corrupted index node read from TNC: danger mode with rebuild_fs and
     normal mode with 'yes' answer will turn to rebuild filesystem, other
     modes will exit.

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