Add INVARIANTS-only fences around lockless vnode refcount updates.

Some internal KASSERTs access the v_iflag field without the vnode
interlock held after such a refcount update. The fences are needed for
the assertions to be correct in the face of store reordering.

Reported and tested by: jhibbits
Reviewed by: kib, mjg
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16756