Fix a bug in UBIFS free space fix-up reported already twice recently:

http://lists.infradead.org/pipermail/linux-mtd/2012-May/041408.html
http://lists.infradead.org/pipermail/linux-mtd/2012-June/042422.html

and we finally have the fix. I am quite confident the fix is correct
because I could reproduce the problem with nandsim and verify the
fix. It was also verified by Iwo (the reporter).

I am also confident that this is OK to merge the fix so late because
this patch affects only the fixup functionality, which is not used by
most users.
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