if_ovpn: add interface counters

Count input/output packets and bytes on the interface as well, not just
in openvpn-specific counters.

PR: 292464
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 21d666a19331f31fb6dfa1e370de5a84a1a5cb46)