pipe: allow passing around of ops private pointer

relay needs this for proper consumption handling, and the network
receive support needs it as well to lookup the sk_buff on pipe
release.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/include/linux/splice.h b/include/linux/splice.h
index f8cc97f..33e447f 100644
--- a/include/linux/splice.h
+++ b/include/linux/splice.h
@@ -41,6 +41,7 @@
 struct partial_page {
 	unsigned int offset;
 	unsigned int len;
+	unsigned long private;
 };
 
 /*