skbuff: make skb_put_zero() return void

It's nicer to return void, since then there's no need to
cast to any structures. Currently none of the users have
a cast, but a number of future conversions do.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1904,9 +1904,9 @@ static inline unsigned char *__skb_put(struct sk_buff *skb, unsigned int len)
 	return tmp;
 }
 
-static inline unsigned char *skb_put_zero(struct sk_buff *skb, unsigned int len)
+static inline void *skb_put_zero(struct sk_buff *skb, unsigned int len)
 {
-	unsigned char *tmp = skb_put(skb, len);
+	void *tmp = skb_put(skb, len);
 
 	memset(tmp, 0, len);