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_vrf b/openflow_input/bsn_vrf
index 3641faa..26959bd 100644
--- a/openflow_input/bsn_vrf
+++ b/openflow_input/bsn_vrf
@@ -41,7 +41,7 @@
 };
 
 struct of_oxm_bsn_vrf_masked : of_oxm {
-    uint32_t type_len == 0x00030504;
+    uint32_t type_len == 0x00030508;
     uint32_t value;
     uint32_t value_mask;
 };