act_nat: fix the wrong checksum when addr isn't in old_addr/mask

fix the wrong checksum when addr isn't in old_addr/mask

For TCP and UDP packets, when addr isn't in old_addr/mask we don't do SNAT or
DNAT, and we should not update layer 4 checksum.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
----
 net/sched/act_nat.c |    4 ++++
 1 file changed, 4 insertions(+)
Signed-off-by: David S. Miller <davem@davemloft.net>