Work around the "nfscl: bad open cnt on server" assertion
that can happen when rerooting into NFSv4 rootfs with kernel
built with INVARIANTS.

I've talked to rmacklem@ (back in 2017), and while the root cause
is still unknown, the case guarded by assertion (nfscl_doclose()
being called from VOP_INACTIVE) is believed to be safe, and the
whole thing seems to run just fine.

Obtained from: CheriBSD
MFC after: 2 weeks
Sponsored by: DARPA, AFRL