ifb: fix building without CONFIG_NET_CLS_ACT

The driver no longer depends on this option, but it fails to
build if it's disabled because the skb->tc_skip_classify is
hidden behind an #ifdef:

drivers/net/ifb.c:81:8: error: no member named 'tc_skip_classify' in 'struct sk_buff'
                skb->tc_skip_classify = 1;

Use the same #ifdef around the assignment.

Fixes: 046178e ("ifb: Depend on netfilter alternatively to tc")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ifb.c

@@ -78,7 +78,9 @@ static void ifb_ri_tasklet(struct tasklet_struct *t)
 	while ((skb = __skb_dequeue(&txp->tq)) != NULL) {
 		/* Skip tc and netfilter to prevent redirection loop. */
 		skb->redirected = 0;
+#ifdef CONFIG_NET_CLS_ACT
 		skb->tc_skip_classify = 1;
+#endif
 		nf_skip_egress(skb, true);
 
 		u64_stats_update_begin(&txp->tsync);