[NET_SCHED]: cls_basic: fix NULL pointer dereference

cls_basic doesn't allocate tp->root before it is linked into the
active classifier list, resulting in a NULL pointer dereference
when packets hit the classifier before its ->change function is
called.

Reported by Chris Madden <chris@reflexsecurity.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

net/sched/cls_basic.c

@@ -83,6 +83,13 @@ static void basic_put(struct tcf_proto *tp, unsigned long f)
 
 static int basic_init(struct tcf_proto *tp)
 {
+	struct basic_head *head;
+
+	head = kzalloc(sizeof(*head), GFP_KERNEL);
+	if (head == NULL)
+		return -ENOBUFS;
+	INIT_LIST_HEAD(&head->flist);
+	tp->root = head;
 	return 0;
 }
 
@@ -178,16 +185,6 @@ static int basic_change(struct tcf_proto *tp, unsigned long base, u32 handle,
 	}
 
 	err = -ENOBUFS;
-	if (head == NULL) {
-		head = kmalloc(sizeof(*head), GFP_KERNEL);
-		if (head == NULL)
-			goto errout;
-
-		memset(head, 0, sizeof(*head));
-		INIT_LIST_HEAD(&head->flist);
-		tp->root = head;
-	}
-
 	f = kmalloc(sizeof(*f), GFP_KERNEL);
 	if (f == NULL)
 		goto errout;