net_sched: act_gact: use a separate packet counters for gact_determ()

Second step for gact RCU operation :

We want to get rid of the spinlock protecting gact operations.
Stats (packets/bytes) will soon be per cpu.

gact_determ() would not work without a central packet counter,
so lets add it for this mode.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>