set relay file can not be read by pread(2)

I found that relay files can be read by pread(2). I fix it,
for relay files are not capable of seeking.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

kernel/relay.c

@@ -736,7 +736,7 @@ static int relay_file_open(struct inode *inode, struct file *filp)
 	kref_get(&buf->kref);
 	filp->private_data = buf;
 
-	return 0;
+	return nonseekable_open(inode, filp);
 }
 
 /**