fsck.ubifs: Add inconsistent problem handling asking function

fsck.ubifs: Add inconsistent problem handling asking function

There are four dimensions to define the type of inconsistent problems:
 1. fixable: Some inconsistent problems can't be fixed, for example
    corrupted superblock. Un-fixable problem will abort program.
 2. must fix: Some inconsistent problems can be ignored(eg. incorrect
    isize), but some are not(eg. corrupted TNC), which will affect the
    subsequent fsck steps.
 3. drop data: Some fixing methods will drop user data, which is
    unacceptable for safe mode. If it happens, fsck will be aborted.
 4. need rebuild: Some inconsistent problems depends on rebuilding
    filesystem to be fixed(eg. corrupted master node, corrupted TNC).
Define an asking function to handle above kinds of inconsistent problems.

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