[NETLINK]: Introduce nlmsg_hdr() helper

For the common "(struct nlmsghdr *)skb->data" sequence, so that we reduce the
number of direct accesses to skb->data and for consistency with all the other
cast skb member helpers.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/connector/connector.c

@@ -212,7 +212,7 @@ static void cn_rx_skb(struct sk_buff *__skb)
 	skb = skb_get(__skb);
 
 	if (skb->len >= NLMSG_SPACE(0)) {
-		nlh = (struct nlmsghdr *)skb->data;
+		nlh = nlmsg_hdr(skb);
 
 		if (nlh->nlmsg_len < sizeof(struct cn_msg) ||
 		    skb->len < nlh->nlmsg_len ||