block: Fix bad data direction in SG_IO

Contrary to what the name misleads you to believe, SG_DXFER_TO_FROM_DEV
is really just a normal read seen from the device side.

This patch fixes http://lkml.org/lkml/2006/10/13/100



Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>