UBIFS: fix extremely rare mount failure

This patch fixes an extremely rare mount failure after a power cut, when mount
fails with ENOSPC error because UBIFS could not find the GC LEB.

In short, the reason for this failure is that after recovery the GC head LEB
contains less free space than it had contained just before the power cut
happened. As a result, if the FS is full, 'ubifs_rcvry_gc_commit()' is unable
to find a dirty LEB to GC and a free LEB, so mount fails.

This patch contains a huge comment with more detailed explanation, please refer
that comment.

Since this is really really rare and unlikely situation, I do not send this
patch to the stable tree, also because it requires a lot of preparation
patches which I did before. So sending this to -stable would be too risky.

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