The path for loopback is:
LOCAL_OUT: conntrack defrags
POST_ROUTING: conntrack refrags
PRE_ROUTING: skip conntrack defrag because skb->nfct != NULL
PRE_ROUTING: NAT gets hit by fragments

Always defrag on loopback if NAT is compiled in.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Rusty Russel <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>