kernel: add a small xfrm related performance optimization

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r41950

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42008 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/generic/patches-3.10/616-net_optimize_xfrm_calls.patch

@@ -0,0 +1,13 @@
+--- a/net/netfilter/nf_nat_core.c
++++ b/net/netfilter/nf_nat_core.c
+@@ -89,6 +89,9 @@ int nf_xfrm_me_harder(struct sk_buff *sk
+ 	struct dst_entry *dst;
+ 	int err;
+ 
++	if (!dev_net(skb->dev)->xfrm.policy_count[XFRM_POLICY_OUT])
++		return 0;
++
+ 	err = xfrm_decode_session(skb, &fl, family);
+ 	if (err < 0)
+ 		return err;
+