net/sched: act_gate: use percpu stats

The tc action act_gate was using shared stats, move it to percpu stats.

tdc results:
1..12
ok 1 5153 - Add gate action with priority and sched-entry
ok 2 7189 - Add gate action with base-time
ok 3 a721 - Add gate action with cycle-time
ok 4 c029 - Add gate action with cycle-time-ext
ok 5 3719 - Replace gate base-time action
ok 6 d821 - Delete gate action with valid index
ok 7 3128 - Delete gate action with invalid index
ok 8 7837 - List gate actions
ok 9 9273 - Flush gate actions
ok 10 c829 - Add gate action with duplicate index
ok 11 3043 - Add gate action with invalid index
ok 12 2930 - Add gate action with cookie

Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>