relay: set an spd_release() hook for splice

relay doesn't reference the pages it adds, however we need a non-NULL
hook or splice_to_pipe() can oops.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

kernel/relay.c

@@ -1056,6 +1056,10 @@ static struct pipe_buf_operations relay_pipe_buf_ops = {
 	.get = generic_pipe_buf_get,
 };
 
+static void relay_page_release(struct splice_pipe_desc *spd, unsigned int i)
+{
+}
+
 /*
  *	subbuf_splice_actor - splice up to one subbuf's worth of data
  */
@@ -1083,6 +1087,7 @@ static int subbuf_splice_actor(struct file *in,
 		.partial = partial,
 		.flags = flags,
 		.ops = &relay_pipe_buf_ops,
+		.spd_release = relay_page_release,
 	};
 
 	if (rbuf->subbufs_produced == rbuf->subbufs_consumed)