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: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>