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_state.c b/net/xfrm/xfrm_state.c
index df93183..e47e498 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -1908,7 +1908,7 @@
 		return -EAFNOSUPPORT;
 	spin_lock_bh(&xfrm_state_afinfo_lock);
 	if (unlikely(xfrm_state_afinfo[afinfo->family] != NULL))
-		err = -ENOBUFS;
+		err = -EEXIST;
 	else
 		rcu_assign_pointer(xfrm_state_afinfo[afinfo->family], afinfo);
 	spin_unlock_bh(&xfrm_state_afinfo_lock);