xfrm: Don't prohibit AH from using ESN feature

Clear checking when user try to use ESN through netlink keymgr for AH.
As only ESP and AH support ESN feature according to RFC.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index 1ae3ec7..ade9988 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -142,7 +142,8 @@
 	if (!rt)
 		return 0;
 
-	if (p->id.proto != IPPROTO_ESP)
+	/* As only ESP and AH support ESN feature. */
+	if ((p->id.proto != IPPROTO_ESP) && (p->id.proto != IPPROTO_AH))
 		return -EINVAL;
 
 	if (p->replay_window != 0)