netfilter: Clean up unnecessary #ifdef

If CONFIG_NETFILTER_INGRESS is not enabled, nf_ingress() becomes a no-op
because it solely contains an if-clause calling nf_hook_ingress_active(),
for which an empty inline stub exists in <linux/netfilter_ingress.h>.

All the symbols used in the if-clause's body are still available even if
CONFIG_NETFILTER_INGRESS is not enabled.

The additional "#ifdef CONFIG_NETFILTER_INGRESS" in nf_ingress() is thus
unnecessary, so drop it.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/core/dev.c

@@ -4932,7 +4932,6 @@ static bool skb_pfmemalloc_protocol(struct sk_buff *skb)
 static inline int nf_ingress(struct sk_buff *skb, struct packet_type **pt_prev,
 			     int *ret, struct net_device *orig_dev)
 {
-#ifdef CONFIG_NETFILTER_INGRESS
 	if (nf_hook_ingress_active(skb)) {
 		int ingress_retval;
 
@@ -4946,7 +4945,6 @@ static inline int nf_ingress(struct sk_buff *skb, struct packet_type **pt_prev,
 		rcu_read_unlock();
 		return ingress_retval;
 	}
-#endif /* CONFIG_NETFILTER_INGRESS */
 	return 0;
 }