if_ovpn: fill out sin_len/sin6_len

When we parse an nvlist sockaddr we should set the sockaddr_in(6)'s length
field. This isn't currently used by anything yet, but it's reasonable to expect
a sockaddr to contain its length.

MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit e83df5367d30761803e09bd7fcf518638dfe43d2)