Make the KASSERT message in hash destroy more informative.

While the pointer might not be too helpful, the malloc type might at
least give a good hint about which hashtbl we are talking.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Reviewed by:	gnn, emaste
Differential Revision:	https://reviews.freebsd.org/D5802

sys/kern/subr_hash.c

@@ -93,7 +93,8 @@ hashdestroy(void *vhashtbl, struct malloc_type *type, u_long hashmask)
 
 	hashtbl = vhashtbl;
 	for (hp = hashtbl; hp <= &hashtbl[hashmask]; hp++)
-		KASSERT(LIST_EMPTY(hp), ("%s: hash not empty", __func__));
+		KASSERT(LIST_EMPTY(hp), ("%s: hashtbl %p not empty "
+		    "(malloc type %s)", __func__, hashtbl, type->ks_shortdesc));
 	free(hashtbl, type);
 }