Linus reports a _really_ small& slow (505kB, 15kB/s) USB device,
on which blkid runs unpleasantly slow. He manages to optimize the blkid
reads down to 1kB+16kB, but still kernel read-ahead turns it into 48kB.
CC: Li Shaohua<shaohua.li@xxxxxxxxx>
CC: Clemens Ladisch<clemens@xxxxxxxxxx>
Acked-by: Jens Axboe<jens.axboe@xxxxxxxxxx>
Tested-by: Vivek Goyal<vgoyal@xxxxxxxxxx>
Tested-by: Linus Torvalds<torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang<fengguang.wu@xxxxxxxxx>