[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/drivers/scsi/scsi_netlink.c b/drivers/scsi/scsi_netlink.c
index 1b59b27..45646a2 100644
--- a/drivers/scsi/scsi_netlink.c
+++ b/drivers/scsi/scsi_netlink.c
@@ -50,7 +50,7 @@
 	while (skb->len >= NLMSG_SPACE(0)) {
 		err = 0;
 
-		nlh = (struct nlmsghdr *) skb->data;
+		nlh = nlmsg_hdr(skb);
 		if ((nlh->nlmsg_len < (sizeof(*nlh) + sizeof(*hdr))) ||
 		    (skb->len < nlh->nlmsg_len)) {
 			printk(KERN_WARNING "%s: discarding partial skb\n",