ubifs-utils: Adapt sb.c in libubifs

ubifs-utils: Adapt sb.c in libubifs

Adapt sb.c in libubifs, compared with linux kernel implementations:
 1. Remove authentication related implementations
    (eg. authenticate_sb_node), authentication is not supported in fsck
    for now.
 2. Remove some functions(eg. create_default_filesystem) which won't be
    used in fsck/mkfs.
 3. Modify ubifs_read_superblock(), remove some assignments which won't
    be used in mkfs/fsck.
 4. Apapt fixup_leb to ignore %-EBADMSG, subsequent steps will check
    all areas carefully.

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