mpls_iptunnel: fix sparse warn: remove incorrect rcu_dereference

fix for:
net/mpls/mpls_iptunnel.c:73:19: sparse: incompatible types in comparison
expression (different address spaces)

remove incorrect rcu_dereference possibly left over from
earlier revisions of the code.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/mpls/mpls_iptunnel.c

@@ -70,7 +70,7 @@ int mpls_output(struct sock *sk, struct sk_buff *skb)
 	skb_orphan(skb);
 
 	/* Find the output device */
-	out_dev = rcu_dereference(dst->dev);
+	out_dev = dst->dev;
 	if (!mpls_output_possible(out_dev) ||
 	    !lwtstate || skb_warn_if_lro(skb))
 		goto drop;