[PKT_SCHED] ematch: tcf_em_destroy robustness

Make the code in tcf_em_tree_destroy more robust and cleaner:
* Don't need to cast pointer to kfree() or avoid passing NULL.
* After freeing the tree, clear the pointer to avoid possible problems
from repeated free.

Signed-off-by: Stephen Hemminger <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sched/ematch.c

···
 		if (em->ops) {
 			if (em->ops->destroy)
 				em->ops->destroy(tp, em);
-			else if (!tcf_em_is_simple(em) && em->data)
-				kfree((void *) em->data);
+			else if (!tcf_em_is_simple(em))
+				kfree(em->data);
 			module_put(em->ops->owner);
 		}
 	}
 
 	tree->hdr.nmatches = 0;
 	kfree(tree->matches);
+	tree->matches = NULL;
 }
 EXPORT_SYMBOL(tcf_em_tree_destroy);