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>
diff --git a/kernel/relay.c b/kernel/relay.c
index 4c035a8..ed3f6cf 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -736,7 +736,7 @@
 	kref_get(&buf->kref);
 	filp->private_data = buf;
 
-	return 0;
+	return nonseekable_open(inode, filp);
 }
 
 /**