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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30585