pf: Avoid leaking pad bytes in struct pfr_astats when copying out

There is padding between pfr_astats.pfras_a and pfras_packets that was
not getting initialized.

Reported by: KMSAN
Reviewed by: kp, imp
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 60a38abb8982e11ee71559057dd7128bd097043e)