openflow_input: fix lengths in masked extension OXMs

The other backends don't rely on these lengths which is why this was never
noticed. The C backend relies on them, but used to set the length field for
OXMs itself.
diff --git a/openflow_input/bsn_lag_id b/openflow_input/bsn_lag_id
index c96c1d6..63645a3 100644
--- a/openflow_input/bsn_lag_id
+++ b/openflow_input/bsn_lag_id
@@ -41,7 +41,7 @@
 };
 
 struct of_oxm_bsn_lag_id_masked : of_oxm {
-    uint32_t type_len == 0x00030304;
+    uint32_t type_len == 0x00030308;
     uint32_t value;
     uint32_t value_mask;
 };