fsck.ubifs: Move common functions and data structures into fsck.ubifs.c

fsck.ubifs: Move common functions and data structures into fsck.ubifs.c

This is a preparation for adding TNC checking support. Following data
structures and functions are moved into fsck.ubifs.c:
 1. Move 'scanned_files' and 'used_lebs' from rebuild module, make them
    resuable for non-rebuild_fs modes.
 2. Move function 'handle_error' from load_fs.c, it could be reused in
    other steps.
 3. Add new function ubifs_tnc_remove_node in libubifs, which could
    remove index entry for a node by given position.

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