xfrm: Mark flowi arg to ->fill_dst() const.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index ef12e68..1e9844d 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -68,7 +68,7 @@
 }
 
 static int xfrm4_fill_dst(struct xfrm_dst *xdst, struct net_device *dev,
-			  struct flowi *fl)
+			  const struct flowi *fl)
 {
 	struct rtable *rt = (struct rtable *)xdst->route;