Fix regression issue after r353274:

Make sure the vnet_shutdown field is not set until after all
VNET_SYSUNINIT()'s in the SI_SUB_VNET_DONE subsystem have been
executed. Especially the vnet_if_return() functions requires that
if_move() is still operational.

Reported by: lwhsu@
MFC after: 1 week
Sponsored by: Mellanox Technologies