Resort the CURVNET_SET* macros in the non-VNET_DEBUG case to match
the call order of the VNET_DEBUG case.
Add the VNET_ASSERT() to the non-VNET_DEBUG case as well so that
INVARIANTS will still catch problems.
Sponsored by: The FreeBSD Foundation
Sponsored by: CK Software GmbH
Reviewed by: jhb
MFC after: 2 weeks