xfrm: fix the return code when xfrm_*_register_afinfo failed

If xfrm_*_register_afinfo failed since xfrm_*_afinfo[afinfo->family] had the
value, return the -EEXIST, not -ENOBUFS

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 847053e..c4c47f3 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2808,7 +2808,7 @@
 		return -EAFNOSUPPORT;
 	spin_lock(&xfrm_policy_afinfo_lock);
 	if (unlikely(xfrm_policy_afinfo[afinfo->family] != NULL))
-		err = -ENOBUFS;
+		err = -EEXIST;
 	else {
 		struct dst_ops *dst_ops = afinfo->dst_ops;
 		if (likely(dst_ops->kmem_cachep == NULL))