Add additional "spare" fields to 'struct ifnet' in order to improve

the resistance of the network driver ABI to changes that will be
required as we optimize locking.

MFC after:	3 days
Discussed at:	Developer Summit

sys/net/if_var.h

@@ -177,6 +177,11 @@ struct ifnet {
 		(void *);
 	int	(*if_resolvemulti)	/* validate/resolve multicast */
 		(struct ifnet *, struct sockaddr **, struct sockaddr *);
+	void	*if_spare1;		/* spare pointer 1 */
+	void	*if_spare2;		/* spare pointer 2 */
+	void	*if_spare3;		/* spare pointer 3 */
+	u_int	if_spare_flags1;	/* spare flags 1 */
+	u_int	if_spare_flags2;	/* spare flags 2 */
 #if 1 /* ALTQ */
 	struct  ifaltq if_snd;		/* output queue (includes altq) */
 #else