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.

Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/scsi/scsi_lib.c

@@ -1018,8 +1018,6 @@ static int scsi_init_sgtable(struct request *req, struct scsi_data_buffer *sdb,
 		return BLKPREP_DEFER;
 	}
 
-	req->buffer = NULL;
-
 	/* 
 	 * Next, walk the list, and fill in the addresses and sizes of
 	 * each segment.
@@ -1156,7 +1154,6 @@ int scsi_setup_blk_pc_cmnd(struct scsi_device *sdev, struct request *req)
 		BUG_ON(blk_rq_bytes(req));
 
 		memset(&cmd->sdb, 0, sizeof(cmd->sdb));
-		req->buffer = NULL;
 	}
 
 	cmd->cmd_len = req->cmd_len;