iflib: Add an admin task detach fail pointHEADmain

Add an exact-device fail point immediately after the admin task checks
IFC_IN_DETACH. This makes the detach race reproducible without affecting
another interface.

Use a bounded delay to keep the task active while detach enters the
taskqueue drain.  Mark the point nonsleepable as a safety backstop, and
document a one-shot test for verifying that deregistration drains an
already-running task before ether_ifdetach().

Reviewed by:	gallatin, kgalazka
MFC after:	2 weeks
Sponsored by:	BBOX.io
Differential Revision:	https://reviews.freebsd.org/D58720

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