[Yaffs] yaffs speed ups

Hi All

Yesterday I checked in some code that speeds up the way yaffs2 calculates the
dirtiest block. This will speed up garbage collection and, therefore,
writing. The speed up is dependent on various factors including the partition
size.

One yaffs user is reporting reasonable write speed improvements of 10-30% on
the partition sizes they are testing and larger partitions will likely give
better improvements.

But wait, there's more.... well not quite yet since it is not checked in
yet... Yesterday also saw me developing some code that defers the writing of
parent directory updates. This helps quite a lot when many small files in a
directory are written in a short period. In the extreme case of doing
something like

for (( f=0; f<1000; f++ ))
    do
        touch file$f
    done
rm file*

the writes are reduced by around 40% giving a good speed up. Of course that is
not typical usage.

That is the first step in moving some activity out of the foreground writing
thread. The next step should give even more speed up.

-- Charles