[PATCH]enhanced wear-leveling based on erase count

[zhao forrest]

Hi,

Here I port my original enhanced WL patch to latest 
MTD CVS.

The main changes are:
1 use the gap between index_of_highest_filled_bucket and
index_of_lowest_filled_bucket as the timing indicator of
WL adjustment.
2 when WL adjustment is necessary, we get block from used
blocks hash table with 1% possiblity.

The patch is at:
http://www.infradead.org/~forrest/wl.patch

Thanks,
Forrest