udp_set_kernel_tunneling() rejects new callbacks if one is already set.
Allow callbacks to be cleared. The use case for this is OpenVPN DCO,
where the socket is opened by userspace and then adopted by the kernel
to run the tunnel. If the DCO interface is removed but userspace does
not close the socket (something the kernel cannot prevent) the installed
callbacks could be called with an invalidated context.

Allow new functions to be set, but only if they're NULL (i.e. allow the
callback functions to be cleared).

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