act_nat: the checksum of ICMP doesn't have pseudo header

after updating the value of the ICMP payload, inet_proto_csum_replace4() should
be called with zero pseudohdr.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>