40_blkdev-pagecache-3

This moves blkdev in pagecache. It synchronizes the buffercache with
pagecache if the blkdev is closed and a ro fs was mounted under it.
It should be safe to use except it obviously breaks the ramdisk driver
very badly (initrd included) so this has to stay in the experimental directory
for now (though I run it on my systems and I expect it to be otherwise
stable). The fix in the ramdisk consists in doing a rewrite of the request
function to lookup on the pagecache instead of the buffercache and I
didn't had the time to do it yet. If there's any volunteer to fix
ramdisk let me know so we don't duplcate efforts.