[PATCH] ll_rw_blk: allow more flexibility for read_ahead_kb store

It can make sense to set read-ahead larger than a single request.
We should not be enforcing such policy on the user. Additionally,
using the BLKRASET ioctl doesn't impose such a restriction. So
additionally we now expose identical behaviour through the two.

Issue also reported by Anton <cbou@mail.ru>

Signed-off-by: Jens Axboe <axboe@suse.de>