act_nat: use stack variable

structure tc_nat isn't too big for stack, so we can put it in stack.
Signed-off-by: Changli Gao <xiaosuo@gmail.com>
----
 net/sched/act_nat.c |   31 ++++++++++---------------------
 1 file changed, 10 insertions(+), 21 deletions(-)
Signed-off-by: David S. Miller <davem@davemloft.net>