libpfctl: Fix u_* counters

struct pf_rule had a few counter_u64_t counters. Those couldn't be
usefully comminicated with userspace, so the fields were doubled up in
uint64_t u_* versions.

Now that we use struct pfctl_rule (i.e. a fully userspace version) we
can safely change the structure and remove this wart.

Reviewed by: glebius
MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29645