vm_object: Assert that overcommit charge is released in the object dtor

Reviewed by:	alc, kib
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35780

sys/vm/vm_object.c

@@ -204,6 +204,9 @@ vm_object_zdtor(void *mem, int size, void *arg)
 	KASSERT(object->type == OBJT_DEAD,
 	    ("object %p has non-dead type %d",
 	    object, object->type));
+	KASSERT(object->charge == 0 && object->cred == NULL,
+	    ("object %p has non-zero charge %ju (%p)",
+	    object, (uintmax_t)object->charge, object->cred));
 }
 #endif