The tc action act_nat was using shared stats and taking the per action
lock in the datapath. Improve it by using percpu stats and rcu.

perf before:
- 10.48% tcf_nat_act
   - 81.83% _raw_spin_lock
        81.08% native_queued_spin_lock_slowpath

perf after:
- 0.48% tcf_nat_act

tdc results:
1..27
ok 1 7565 - Add nat action on ingress with default control action
ok 2 fd79 - Add nat action on ingress with pipe control action
ok 3 eab9 - Add nat action on ingress with continue control action
ok 4 c53a - Add nat action on ingress with reclassify control action
ok 5 76c9 - Add nat action on ingress with jump control action
ok 6 24c6 - Add nat action on ingress with drop control action
ok 7 2120 - Add nat action on ingress with maximum index value
ok 8 3e9d - Add nat action on ingress with invalid index value
ok 9 f6c9 - Add nat action on ingress with invalid IP address
ok 10 be25 - Add nat action on ingress with invalid argument
ok 11 a7bd - Add nat action on ingress with DEFAULT IP address
ok 12 ee1e - Add nat action on ingress with ANY IP address
ok 13 1de8 - Add nat action on ingress with ALL IP address
ok 14 8dba - Add nat action on egress with default control action
ok 15 19a7 - Add nat action on egress with pipe control action
ok 16 f1d9 - Add nat action on egress with continue control action
ok 17 6d4a - Add nat action on egress with reclassify control action
ok 18 b313 - Add nat action on egress with jump control action
ok 19 d9fc - Add nat action on egress with drop control action
ok 20 a895 - Add nat action on egress with DEFAULT IP address
ok 21 2572 - Add nat action on egress with ANY IP address
ok 22 37f3 - Add nat action on egress with ALL IP address
ok 23 6054 - Add nat action on egress with cookie
ok 24 79d6 - Add nat action on ingress with cookie
ok 25 4b12 - Replace nat action with invalid goto chain control
ok 26 b811 - Delete nat action with valid index
ok 27 a521 - Delete nat action with invalid index
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>