When creating a pseudo interface we also create a device_t with which
the iflib context is associated. I believe this is done just so iflib
can consistently pass around a device_t.

In commit 38bfc6dee33b we added an IFDI_DETACH() call to
iflib_pseudo_deregister() since it looked like it was missing. One is
present in the error handling path of iflib_pseudo_register(). However,
the detach actually comes from the DEVICE_DETACH() method for the
above-mentioned device_t, so now we're calling IFDI_DETACH() twice when
destroying a pseudo interface.

Fix the problem by not calling IFDI_DETACH() from the device detach
routine. This way we can ensure that iflib de-initialization always
happens in a consistent order. It also ensures that you can't do silly
things like "devctl detach <pseudo ifnet>", which currently panics the
system.