if_ovpn: cleanup offsetof() use

Move the use of the `offsetof(struct ovpn_counters, fieldname) /
sizeof(uint64_t)` construct into a macro.
This removes a fair bit of code duplication and should make things a
little easier to read.

Reviewed by: zlei
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37607