OPTIM: stream-int: try to send pending spliced data

This is the equivalent of eb9fd51 ("OPTIM: stream_sock: reduce the amount
of in-flight spliced data") whose purpose is to try to immediately send
spliced data if available.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index 781f449..bafb0d2 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -180,7 +180,8 @@
 		si_chk_rcv(si->ob->prod);
 
 	if (((si->ib->flags & CF_READ_PARTIAL) && !channel_is_empty(si->ib)) &&
-	    (si->ib->cons->flags & SI_FL_WAIT_DATA)) {
+	    (si->ib->pipe /* always try to send spliced data */ ||
+	     (si->ib->buf->i == 0 && (si->ib->cons->flags & SI_FL_WAIT_DATA)))) {
 		si_chk_snd(si->ib->cons);
 		/* check if the consumer has freed some space */
 		if (!channel_full(si->ib) && !si->ib->pipe)