sbull: fix

root cause commits in linux-stable:
commit b4f42e2831ff9b9fa19252265d7c8985d47eefb9
Author: Jens Axboe <[email protected]>
Date:   Thu Apr 10 09:46:28 2014 -0600

    block: remove struct request buffer member

    This was used in the olden days, back when onions were proper
    yellow. Basically it mapped to the current buffer to be
    transferred. With highmem being added more than a decade ago,
    most drivers map pages out of a bio, and rq->buffer isn't
    pointing at anything valid.

    Convert old style drivers to just use bio_data().

    For the discard payload use case, just reference the page
    in the bio.

sbull/sbull.c

@@ -127,7 +127,7 @@ static void sbull_request(struct request_queue *q)
 			(unsigned)(dev - Devices), rq_data_dir(req),
 			blk_rq_pos(req), blk_rq_cur_sectors(req));
 		sbull_transfer(dev, blk_rq_pos(req), blk_rq_cur_sectors(req),
-				req->buffer, rq_data_dir(req));
+				bio_data(req->bio), rq_data_dir(req));
 		ret = 0;
 	done:
 		if(!__blk_end_request_cur(req, ret)){