The netlink encoding of struct pfr_addr omits pfra_fback, so no
per-address feedback reaches userspace. In particular,
pfr_get_astats() marks entries of tables without counters with
PFR_FB_NOCOUNT, which pfctl uses to skip their counters, so
"pfctl -v -T show" prints zero counters for such tables.
Add PFR_A_FBACK, emit it from the kernel and decode it in libpfctl.
Add a regression test.
Fixes: 08f54dfca197 ("pf: convert DIOCRGETASTATS to netlink")
Sponsored by: Rubicon Communications, LLC ("Netgate")