lwtunnel: subtract tunnel headroom from mtu on output redirect

This patch changes the lwtunnel_headroom() function which is called
in ipv4_mtu() and ip6_mtu(), to also return the correct headroom
value when the lwtunnel state is OUTPUT_REDIRECT.

This patch enables e.g. SR-IPv6 encapsulations to work without
manually setting the route mtu.
Acked-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NDavid Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/lwtunnel.h

@@ -94,7 +94,8 @@ static inline bool lwtunnel_xmit_redirect(struct lwtunnel_state *lwtstate)
 static inline unsigned int lwtunnel_headroom(struct lwtunnel_state *lwtstate,
 					     unsigned int mtu)
 {
-	if (lwtunnel_xmit_redirect(lwtstate) && lwtstate->headroom < mtu)
+	if ((lwtunnel_xmit_redirect(lwtstate) ||
+	     lwtunnel_output_redirect(lwtstate)) && lwtstate->headroom < mtu)
 		return lwtstate->headroom;
 
 	return 0;