pseudofs: Don't purge the cache on shutdown

[des]

Summary

This is a waste of time and results in a use-after-free if linsysfs is
loaded and a USB network interface is in use, since USB devices are
disconnected at shutdown, which triggers a call into linsysfs, which
then tries to destroy a pseudofs node which has already been purged.

MFC after: 1 week
Reported by: glebius

[glebius]

Thanks! While I'm 99% sure it is going to fix my panic, I can't test it right away. Traveling right now and machine that exhibits it is not with me. I'll be able to test next week. If you want you can delay the push until I test. I will report here, in this revision.

[des]

@glebius have you been able to test?

[glebius]

I will be able tomorrow. Was traveling this week. Will update.

[glebius]

Yes, the patch works as expected: fixes the panic on shutdown. Thanks!