libpfctl: Fix u_* counters

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

(cherry picked from commit ab5707a5cf86ed66dbd11e03ad9676766da9a47b)