UBIFS: make force in-the-gaps to be a general self-check

UBIFS can force itself to use the 'in-the-gaps' commit method - the last resort
method which is normally invoced very very rarely. Currently this "force
int-the-gaps" debugging feature is a separate test mode. But it is a bit saner
to make it to be the "general" self-test check instead.

This patch is just a clean-up which should make the debugging code look a bit
nicer and easier to use - we have way too many debugging options.

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

Documentation/filesystems/ubifs.txt

@@ -154,7 +154,6 @@ debug_tsts	Selects a mode of testing, as follows:
 
 		Test mode				Flag value
 
-		Force in-the-gaps method		2
 		Failure mode for recovery testing	4
 
 For example, set debug_msgs to 5 to display General messages and Mount