Panic in kobj_delete() initiated by uhub detachment.

[Sangwoo Shim]

Hello.
Several people reported uhub-related panic to -current mailing list.
Detaching uhub with some device attached to it is enough to reproduce the panic
in sufficiently up-to-date CURRENT and RELENG_5.

I've submitted PR kern/71956, which includes DDB and kgdb output.
http://www.freebsd.org/cgi/query-pr.cgi?pr=71956

Regards,
Sangwoo Shim