cxgbe(4): Fix bad races between sysctl and driver detach.
The default sysctl context setup by newbus for a device is eventually freed by device_sysctl_fini, which runs after the device driver's detach routine. sysctl nodes associated with this context must not use any resources (like driver locks, hardware access, counters, etc.) that are released by driver detach. There are a lot of sysctl nodes like this in cxgbe(4) and the fix is to hang them off a context that is explicitly freed by the driver before it releases any resource that might be used by a sysctl. This fixes panics when running "sysctl dev.t6nex dev.cc" in a tight loop and loading/unloading the driver in parallel. Reported by: Suhas Lokesha MFC after: 1 week Sponsored by: Chelsio Communications