pfsync: Avoid zeroing the state export union

pfsync_state_export() takes a pointer to a union that is in reality a
pointer to one of the three state formats (1301, 1400, 1500), and zeros
the union. The three formats do not have the same size, so zeroing is
wrong when the format isn't that which has the largest size.

Refactor a bit so that the zeroing happens at the layer where we know
which format we're dealing with.

Reported by: CHERI
Reviewed by: kp
MFC after: 1 week
Sponsored by: CHERI Research Centre (EPSRC grant UKRI3001)
Differential Revision: https://reviews.freebsd.org/D54163