iflib: Fix detach of pseudo interfaces

iflib: Fix detach of pseudo interfaces

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 would previously
detach the driver without tearing down the corresponding ifnet.

PR:		253541
Reviewed by:	erj
MFC after:	1 week
Fixes:		38bfc6dee33b
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D28774