ubifs-utils: Adapt debug subsystem in libubifs

ubifs-utils: Adapt debug subsystem in libubifs

Adapt debug subsystem(debug.c, debug.h) in libubifs, compared with
linux kernel implementations:
 1. Only keep the disk data and space statistics dumping implementations,
    dbg_walk_index and add_size which are used by fsck, other debuging
    related implementations and sysfs interfaces are removed, because
    fsck will check fs in another way.
 2. Change the behavior of ubifs_assert_failed(), make filesystem
    readonly when assertion is failed.

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