fsck.ubifs: rebuild_fs: Filter invalid files

fsck.ubifs: rebuild_fs: Filter invalid files

This is the 4/12 step of rebuilding. Filter out invalid files and drop
them, for example:
 1. File has no inode node or inode nlink is zero
 2. Nonconsistent file type between inode node and dentry nodes
 3. File has no dentry nodes(excepts '/')
 4. Encrypted file has no xattr information
 5. Non regular file has data nodes
 6. Directory/xattr file has more than one dentries
 7. Xattr file has no host inode, or the host inode is a xattr
 ...
Valid xattr file will be inserted into corresponding host file's subtree.

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