[NETFILTER]: nf_conntrack_ipv4: fix incorrect #ifdef config name

The option is named CONFIG_NF_NAT not CONFIG_IP_NF_NAT. Remove the ifdef
completely since helpers also expect defragmented packet even without
NAT.

Noticed by Robert P. J. Day <[e-mail]>

Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c

···
 					  const struct net_device *out,
 					  int (*okfn)(struct sk_buff *))
 {
-#if !defined(CONFIG_IP_NF_NAT) && !defined(CONFIG_IP_NF_NAT_MODULE)
 	/* Previously seen (loopback)?  Ignore.  Do this before
 	   fragment check. */
 	if ((*pskb)->nfct)
 		return NF_ACCEPT;
-#endif
 
 	/* Gather fragments. */
 	if (ip_hdr(*pskb)->frag_off & htons(IP_MF | IP_OFFSET)) {