[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_transport_iscsi.c b/drivers/scsi/scsi_transport_iscsi.c
index ce0d14a..10590cd 100644
--- a/drivers/scsi/scsi_transport_iscsi.c
+++ b/drivers/scsi/scsi_transport_iscsi.c
@@ -1081,7 +1081,7 @@
 			struct nlmsghdr	*nlh;
 			struct iscsi_uevent *ev;
 
-			nlh = (struct nlmsghdr *)skb->data;
+			nlh = nlmsg_hdr(skb);
 			if (nlh->nlmsg_len < sizeof(*nlh) ||
 			    skb->len < nlh->nlmsg_len) {
 				break;