sg: set dxferp to NULL for READ with the older SG interface

With the older SG interface, we don't know a user-space address to
trasfer data when executing a SCSI command. So we can't pass a
user-space address to blk_rq_map_user.

This patch fixes sg to pass a NULL user-space address to
blk_rq_map_user so that it just sets up a request and bios with page
frames propely without data transfer.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

drivers/scsi/sg.c

@@ -615,7 +615,10 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
 	else
 		hp->dxfer_direction = (mxsize > 0) ? SG_DXFER_FROM_DEV : SG_DXFER_NONE;
 	hp->dxfer_len = mxsize;
-	hp->dxferp = (char __user *)buf + cmd_size;
+	if (hp->dxfer_direction == SG_DXFER_TO_DEV)
+		hp->dxferp = (char __user *)buf + cmd_size;
+	else
+		hp->dxferp = NULL;
 	hp->sbp = NULL;
 	hp->timeout = old_hdr.reply_len;	/* structure abuse ... */
 	hp->flags = input_size;	/* structure abuse ... */