PR: 168520 170096
Submitted by: adrian, zec

Fix multiple kernel panics when VIMAGE is enabled in the kernel.
These fixes are based on patches submitted by Adrian Chadd and Marko Zec.

(1)  Set curthread->td_vnet to vnet0 in device_probe_and_attach() just before calling
     device_attach().  This fixes multiple VIMAGE related kernel panics
     when trying to attach Bluetooth or USB Ethernet devices because
     curthread->td_vnet is NULL.

(2)  Set curthread->td_vnet in if_detach().  This fixes kernel panics when detaching networking
     interfaces, especially USB Ethernet devices.

(3)  Use VNET_DOMAIN_SET() in ng_btsocket.c

(4)  In ng_unref_node() set curthread->td_vnet.  This fixes kernel panics
     when detaching Netgraph nodes.
PR: kern/168520

Revert GRN 253255.  It is not needed now that GRN 253346 is committed.