[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>
diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index 4c3476f..ad7d239 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -102,7 +102,7 @@
  */
 static int send_reply(struct sk_buff *skb, pid_t pid)
 {
-	struct genlmsghdr *genlhdr = nlmsg_data((struct nlmsghdr *)skb->data);
+	struct genlmsghdr *genlhdr = nlmsg_data(nlmsg_hdr(skb));
 	void *reply = genlmsg_data(genlhdr);
 	int rc;
 
@@ -121,7 +121,7 @@
 static void send_cpu_listeners(struct sk_buff *skb,
 					struct listener_list *listeners)
 {
-	struct genlmsghdr *genlhdr = nlmsg_data((struct nlmsghdr *)skb->data);
+	struct genlmsghdr *genlhdr = nlmsg_data(nlmsg_hdr(skb));
 	struct listener *s, *tmp;
 	struct sk_buff *skb_next, *skb_cur = skb;
 	void *reply = genlmsg_data(genlhdr);