readahead statistics slimmed down

This patch set simplifies the statistics code of the adaptive readahead feature by using the proposed statistic infrastructure.

Patches are against 2.6.21-rc7-mm2.

[RFC] [Patch 1/3] readahead statistics slimmed down, statistics prereq [RFC] [Patch 2/3] readahead statistics slimmed down, adapt zfcp [RFC] [Patch 3/3] readahead statistics slimmed down