Walk back r337554 while discussion continues

The idea was to get the uncontroversial mechanical change out of the way,
then get the meatier functional changes reviewed subsequently. I had not
realized that the immediately adjacent issue was addressed in a different
direction in r334506 (see Warner's guidance in D15592).

Discussion continues, trying to determine if there is a secondary issue
still[1] and how best to fix it. With 12-related activities coming up,
while that is ongoing, just take this back for now.

[1]: Shutdown-time eventhandler events fire normally during panic's reboot
path. Driver callbacks that attempt to issue and wait on interrupt-
completed IO may never complete, hanging the system. This is particularly
obnoxious in the shutdown/panic path, as the debugger cannot be entered
anymore and the hang prevents reboot restoring availability.

(There's nothing CAM-specific about this problem -- any shutdown
event-triggered driver could do something like this during panic. But most
NICs, etc. don't try to send spin-down commands at shutdown. ;-))

Discussed with: imp, markj