[Yaffs] Some recent fixes to yaffs1 mode

Note: The following only applies to yaffs1 mode of operation.

Other than stuff already reported on the list I found a problem in yaffs1 mode
to do with soft deletion. If soft deletion was enabled then under some really
hard abuse file entries could come back after being deleted.

The fix did not sort this out completely, but it is vastly improved. My stress
tests were showing a problem once in a thousand cycles or so. Now it is only
happening once in 300,000 cycles or so.

Each cycle of these tests consists of running some random-ish and known
operations in parallel then simulating a power failure during a write, then
testing the known file set, and some other state, for any problems. This is
way worse than most typical real world power failures which would tend to
happen at "safe" periods.

This problem does not destroy data. If just presents an extra file entry in a
directory. Thus, it is reasonably benign and hard to make happen.

If you are concerned that this might be an issue then it is possible to
disable soft deletion by setting dev->params.disableSoftDelete=1

Disabling soft deletion will slow down file deletion.

I'm running more tests and trying to figure this problem out.

-- Charles