packet: Fix build with INET disabled.

af_packet.c:(.text+0x3d130): undefined reference to `ip_defrag'
or
ERROR: "ip_defrag" [net/packet/af_packet.ko] undefined!

Reported-by: Randy Dunlap <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/packet/af_packet.c

···
 
 static struct sk_buff *fanout_check_defrag(struct sk_buff *skb)
 {
+#ifdef CONFIG_INET
 	const struct iphdr *iph;
 	u32 len;
 
···
 			skb->rxhash = 0;
 		}
 	}
+#endif
 	return skb;
 }