Use gen_linktype() for VLAN EtherType check.

Directly using off_linktype to match the VLAN EtherType only
works for basic Ethernet cases. Instead we can use gen_linktype()
which is more general and is more likely to be updated to handle
future protocols.

CREDITS

@@ -84,6 +84,7 @@ Additional people who have contributed patches:
     Jefferson Ogata               <jogata at nodc dot noaa dot gov>
     Jesper Dangaard Brouer        <hawk at comx dot dk>
     Jesper Peterson               <jesper at endace dot com>
+    Jesse Gross                   <jesse at nicira dot com>
     Jiri Slaby                    <jirislaby at gmail dot com>
     Joerg Mayer                   <jmayer at loplof dot de>
     John Bankier                  <jbankier at rainfinity dot com>

gencode.c

@@ -8082,10 +8082,8 @@ gen_vlan_no_bpf_extensions(int vlan_num)
         struct block *b0, *b1;
 
         /* check for VLAN, including QinQ */
-        b0 = gen_cmp(OR_LINK, off_linktype, BPF_H,
-                     (bpf_int32)ETHERTYPE_8021Q);
-        b1 = gen_cmp(OR_LINK, off_linktype, BPF_H,
-                     (bpf_int32)ETHERTYPE_8021QINQ);
+        b0 = gen_linktype(ETHERTYPE_8021Q);
+        b1 = gen_linktype(ETHERTYPE_8021QINQ);
         gen_or(b0,b1);
         b0 = b1;