[PATCH] readahead: simplify recent fixes

[PATCH] readahead: simplify recent fixes

Ok I have enclosed the results for the recent readahead fixes.  The summary
is: there is no significant improvement or decrease in performance of (DSS
workload, iozone, sysbench) The increase or decrease is in the margin of
errors.

I have enclosed a patch that partially backs off Miklos's fix.  Shane
Shrybman correctly pointed out that the real fix is to set ra->average
value to max/2 when we move from readahead-off mode to readahead-on mode.
The other part of Miklos's fix becomes irrelevent.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>