if_ovpn: cope with loops

User misconfiguration may lead to routing loops where we try to send the tunnel
packet into the tunnel. This eventually leads to stack overflows and panics.

Avoid this using if_tunnel_check_nesting(), which will drop the packet if we're
looping or we hit three layers of nested tunnels.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")