netinet/if_ether.h: add ETH_P_DSA_8021Q from linux v5.2

ethertype for fake VLAN header for DSA, see

  linux commit bf5bc3ce8a8f32a0d45b6820ede8f9fc3e9c23df
  ether: Add dedicated Ethertype for pseudo-802.1Q DSA tagging

include/netinet/if_ether.h

@@ -76,6 +76,7 @@
 #define ETH_P_QINQ2	0x9200
 #define ETH_P_QINQ3	0x9300
 #define ETH_P_EDSA	0xDADA
+#define ETH_P_DSA_8021Q	0xDADB
 #define ETH_P_IFE	0xED3E
 #define ETH_P_AF_IUCV	0xFBFB