net_sched: gen_estimator: extend packet counter to 64bit

I forgot to change last_packets field in struct net_rate_estimator.

Without this fix, rate estimators would misbehave after more
than 2^32 packets have been sent.

Another solution would be to be careful and only use the
32 least significant bits of packets counters, but we have
a hole in net_rate_estimator structure and this looks
easier to read/maintain.

Fixes: d0083d98

 ("net_sched: extend packet counter to 64bit")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>