if_ovpn: include peer counters in a OVPN_NOTIF_DEL_PEER message

When we remove a peer userspace can no longer retrieve its counters. To
ensure that userspace can get a full count of the entire session we now
include the counters in the deletion message.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37606