UBIFS: lessen amount of debugging check types

We have too many different debugging checks - lessen the amount by merging all
index-related checks into one. At the same time, move the "force in-the-gap"
test to the "index checks" class, because it is too heavy for the "general"
class.

This patch merges TNC, Old index, and Index size check and calles this just
"index checks".

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>