BUG/MINOR: buffers: MT_LIST_DEL_SAFE() expects the temporary pointer.

When calling MT_LIST_DEL_SAFE(), give him the temporary pointer "tmpelt",
as that's what is expected. We want to be able to set that pointer to NULL,
to let other parts of the code know we deleted an element.

src/buffer.c

@@ -120,7 +120,7 @@ void __offer_buffer(void *from, unsigned int threshold)
 		if (wait->target == from || !wait->wakeup_cb(wait->target))
 			continue;
 
-		MT_LIST_DEL_SAFE(&wait->list);
+		MT_LIST_DEL_SAFE(elt1);
 		avail--;
 	}
 }