netfilter: restore lost #ifdef guarding defrag exception

Nir Tzachar <nir.tzachar@gmail.com> reported a warning when sending
fragments over loopback with NAT:

[ 6658.338121] WARNING: at net/ipv4/netfilter/nf_nat_standalone.c:89 nf_nat_fn+0x33/0x155()

The reason is that defragmentation is skipped for already tracked connections.
This is wrong in combination with NAT and ip_conntrack actually had some ifdefs
to avoid this behaviour when NAT is compiled in.

The entire "optimization" may seem a bit silly, for now simply restoring the
lost #ifdef is the easiest solution until we can come up with something better.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/netfilter/nf_defrag_ipv4.c

@@ -41,12 +41,13 @@ static unsigned int ipv4_conntrack_defrag(unsigned int hooknum,
 					  int (*okfn)(struct sk_buff *))
 {
 #if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
+#if !defined(CONFIG_NF_NAT) && !defined(CONFIG_NF_NAT_MODULE)
 	/* Previously seen (loopback)?  Ignore.  Do this before
 	   fragment check. */
 	if (skb->nfct)
 		return NF_ACCEPT;
 #endif
-
+#endif
 	/* Gather fragments. */
 	if (ip_hdr(skb)->frag_off & htons(IP_MF | IP_OFFSET)) {
 		if (nf_ct_ipv4_gather_frags(skb,