iflib: Add an admin task detach fail point

[kbowling]

Summary

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 because the taskqueue lock remains held while the
task runs.  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().

MFC after:      2 weeks

Test Plan

sysctl debug.fail_point.iflib.admin_task_device=igc1
sysctl debug.fail_point.iflib.admin_task_after_detach_check='1*print(1)->1*delay(5000000)'

You will see: fail point admin_task_after_detach_check executing

Then immediately run:
time devctl detach igc1

[gallatin]

This is really cool. I had never know about fail(9)

[nick_spun.io]

In D58720#1348789, @gallatin wrote:

This is really cool. I had never know about fail(9)

Me neither! Would've saved me a ton of injecting faults during some driver work

[kgalazka]

This is great! Both as a patch and as an advertisement for the fail(9). Thank you!