sg: fix a warning in blk_rq_aligned() call

2nd argument of blk_rq_aligned() has changed to 'unsigned long' by
the previous commit 'block: fix an address space warning in blk-map.c'.
That commit neglected to update a user of that function.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>