[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/net/ipv4/inet_diag.c b/net/ipv4/inet_diag.c
index 37362cd..238999e 100644
--- a/net/ipv4/inet_diag.c
+++ b/net/ipv4/inet_diag.c
@@ -847,7 +847,7 @@
 {
 	if (skb->len >= NLMSG_SPACE(0)) {
 		int err;
-		struct nlmsghdr *nlh = (struct nlmsghdr *)skb->data;
+		struct nlmsghdr *nlh = nlmsg_hdr(skb);
 
 		if (nlh->nlmsg_len < sizeof(*nlh) ||
 		    skb->len < nlh->nlmsg_len)