net/skbuff: add macros for VLAN_PRESENT bit

Wrap VLAN_PRESENT bit using macro like PKT_TYPE_* and CLONED_*,
as used by BPF code.
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -816,6 +816,12 @@ struct sk_buff {
 	__u32			priority;
 	int			skb_iif;
 	__u32			hash;
+#define PKT_VLAN_PRESENT_BIT	4	// CFI (12-th bit) in TCI
+#ifdef __BIG_ENDIAN
+#define PKT_VLAN_PRESENT_OFFSET()	offsetof(struct sk_buff, vlan_tci)
+#else
+#define PKT_VLAN_PRESENT_OFFSET()	(offsetof(struct sk_buff, vlan_tci) + 1)
+#endif
 	__be16			vlan_proto;
 	__u16			vlan_tci;
 #if defined(CONFIG_NET_RX_BUSY_POLL) || defined(CONFIG_XPS)