xfrm: Mark flowi arg to ->get_tos() 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 19fbdec..ef12e68 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -56,7 +56,7 @@
 	return 0;
 }
 
-static int xfrm4_get_tos(struct flowi *fl)
+static int xfrm4_get_tos(const struct flowi *fl)
 {
 	return IPTOS_RT_MASK & fl->fl4_tos; /* Strip ECN bits */
 }