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>
diff --git a/kernel/relay.c b/kernel/relay.c
index ed3f6cf..d6204a4 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -1056,6 +1056,10 @@
 	.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 @@
 		.partial = partial,
 		.flags = flags,
 		.ops = &relay_pipe_buf_ops,
+		.spd_release = relay_page_release,
 	};
 
 	if (rbuf->subbufs_produced == rbuf->subbufs_consumed)