Some workloads really, really want to have no readahead.  Databases which are
perfoming small synchronous I/Os against a file which has extremely poor
layout.  Any readahead at all is a lose here.

But the current readahead code refuses to adapt that low.

Fix it up so that we can indeed adaptively disable readahead altogether, and
do not start it again until we have seen max_readahead()'s worth of
consecutive reads.