if_ovpn: fix memory leak on unload

When we're unloading the if_ovpn module we sometimes end up only freeing
the softc after the module is unloaded and the M_OVPN malloc type no
longer exists.

Don't return from ovpn_clone_destroy() until the epoch callbacks have
been called, which ensures that we've freed the softc before we destroy
M_OVPN.

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