MFC r262142:

   In ue_attach_post_task(), initialize curvnet to vnet0 before calling if_attach().
   Before this patch, curvnet was NULL.
   When the VIMAGE kernel option is enabled, this eliminates
   kernel panics when USB ethernet devices are plugged in.

PR: 183835
Submitted by: Hiroo Oono <hiroo.ono at gmail dot com>