net: dsa: tag_ar9331: Make sure there is headroom for tag

Passing tag size to skb_cow_head will make sure
there is enough headroom for the tag data.
This change does not introduce any overhead in case there
is already available headroom for tag.
Signed-off-by: Per Forlin <perfn@axis.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_ar9331.c

@@ -31,7 +31,7 @@ static struct sk_buff *ar9331_tag_xmit(struct sk_buff *skb,
 	__le16 *phdr;
 	u16 hdr;
 
-	if (skb_cow_head(skb, 0) < 0)
+	if (skb_cow_head(skb, AR9331_HDR_LEN) < 0)
 		return NULL;
 
 	phdr = skb_push(skb, AR9331_HDR_LEN);