From: Jens Axboe <axboe@suse.de>

Expose the blockdev's VM readahead in /sys/block/hda/queue/read_ahead_kbytes

This duplicates `blockdev --setra', but we're trying to get away from
ioctls.

It would be nice to have a readahead-setting mechanism which also allows,
say, NFS to be tuned.  But there is no common exposure point for
backing_dev_infos.  One option might be per-superblock:

	mount -o remount,read_ahead_kbytes=64

but the generic remount code also has no visibility of the backing_dev, so it
would need a new super_block operation.  One which doesn't accidentally modify
default_backing_dev_info.