[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/security/selinux/hooks.c b/security/selinux/hooks.c
index addb585..5f02b4b 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3786,7 +3786,7 @@
 		err = -EINVAL;
 		goto out;
 	}
-	nlh = (struct nlmsghdr *)skb->data;
+	nlh = nlmsg_hdr(skb);
 	
 	err = selinux_nlmsg_lookup(isec->sclass, nlh->nlmsg_type, &perm);
 	if (err) {