Fix a panic which (reportedly) can happen when unmounting a filesystem

with I/O requests in flight on kernels compiled with "options INVARIANTS".
Also, make it obvious it's not right to call g_valid_obj() (and macros
using it, e.g. G_VALID_CONSUMER()) without topology lock held.

Approved by:	re (kib)
Reported by:	pho

sys/geom/geom_subr.c

@@ -1025,6 +1025,8 @@ g_valid_obj(void const *ptr)
 	struct g_consumer *cp;
 	struct g_provider *pp;
 
+	g_topology_assert();
+
 	LIST_FOREACH(mp, &g_classes, class) {
 		if (ptr == mp)
 			return (1);

sys/geom/geom_vfs.c

@@ -106,7 +106,7 @@ g_vfs_strategy(struct bufobj *bo, struct buf *bp)
 	int vfslocked;
 
 	cp = bo->bo_private;
-	G_VALID_CONSUMER(cp);
+	/* G_VALID_CONSUMER(cp); We likely lack topology lock */
 
 	/*
 	 * If the the provider has orphaned us, just return EXIO.