darwin knet avoid rx socket during shutdown

darwin knet avoid rx socket during shutdown

theoretically a user thread can race us during shutdown and end up
panicing us if the soclose happens at the wrong time. if we're shutting down
just return. using afs_shuttingdown would have the side effect that we'd lose
the ability to add any graceful behavior.

Reviewed-on: http://gerrit.openafs.org/443
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>