On the false -> true transition of sc_enxio_active, report that we're
doing this. This is a rare, but interesting, evevnt.Convert to using
atomics to set/load this field to prevent a rare race:
In CAM, one thread calls biodone with ENXIO as the return code and another thread can invalidate the device (causing the queued requests to complete with ENXIO). These two threads could race and could lose the printf. Since this is an error path that's infrequent, the extra atomic traffic won't slow down the slow path.
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