pfsync: Fix offset calculation

[vegeta_tuxpowered.net]

Summary

Even though message version is automatically recognized and the top of
the struct is identical for different versions, when iterating over
multiple messages proper message length must be used. That's the length
of an union member for given version, not of the union itself.

Test Plan

Checked logs with pf debug set to loud on my own setup of
FreeBSD 13.2 and 14.0 routers

[vegeta_tuxpowered.net]

pfsync: Fix offset calculation

Summary: Even though message version is automatically recognized and the top of
the struct is identical for different versions, when iterating over
multiple messages proper message length must be used. That's the length
of an union member for given version, not of the union itself.

Test Plan:
Checked logs with pf debug set to loud on my own setup of
FreeBSD 13.2 and 14.0 routers

Subscribers: imp, farrokhi, melifaro, glebius

Differential Revision: https://reviews.freebsd.org/D43862