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

sys/fs/nfsclient/nfs_clstate.c

@@ -3188,8 +3188,11 @@ nfscl_doclose(vnode_t vp, struct nfsclclient **clpp, NFSPROC_T *p)
 			    !NFSBCMP(op->nfso_fh, nfhp->nfh_fh,
 			    nfhp->nfh_len)) {
 				/* Found an open, close it. */
+#ifdef DIAGNOSTIC
 				KASSERT((op->nfso_opencnt == 0),
-				    ("nfscl: bad open cnt on server"));
+				    ("nfscl: bad open cnt on server (%d)",
+				     op->nfso_opencnt));
+#endif
 				NFSUNLOCKCLSTATE();
 				nfsrpc_doclose(VFSTONFS(vnode_mount(vp)), op,
 				    p);