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

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c
index 753e9a1..f2fa904 100644
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c
@@ -87,7 +87,7 @@
 }
 
 static int xfrm6_fill_dst(struct xfrm_dst *xdst, struct net_device *dev,
-			  struct flowi *fl)
+			  const struct flowi *fl)
 {
 	struct rt6_info *rt = (struct rt6_info*)xdst->route;