MINOR: stream-interface: add an rcv_buf callback to sock_ops

This one is to be used by the read I/O handlers.
diff --git a/src/raw_sock.c b/src/raw_sock.c
index f5b5f2f..7dd90c9 100644
--- a/src/raw_sock.c
+++ b/src/raw_sock.c
@@ -343,7 +343,7 @@
 			break;
 		}
 
-		ret = raw_sock_to_buf(conn, &b->buf, max);
+		ret = conn->data->rcv_buf(conn, &b->buf, max);
 		if (ret <= 0)
 			break;
 
@@ -634,6 +634,7 @@
 	.read    = sock_raw_read,
 	.write   = si_conn_send_cb,
 	.snd_buf = sock_raw_write_loop,
+	.rcv_buf = raw_sock_to_buf,
 	.close   = NULL,
 };