g_vfs_done: Report when we switch on ENXIO conversion

[imp]

Summary

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.

Sponsored by: Netflix

[kib]

sys/geom/geom_vfs.c

147

Same note about the race.

[imp]

Fix race

[imp]

sys/geom/geom_vfs.c

147

fixed.

[imp]

sys/geom/geom_vfs.c

193

Not sure if we tag these with atomic_load_int() or not.
In this case, it doesn't matter: if we win the race, we'll have one more request go down and back up and then fail. If we lose the race, we'll fail it here.

[imp]

spell cmpset function right <blush>