[IPSEC]: Add xfrm_state_mtu()

This patch introduces the function xfrm_state_mtu which calculates
the MTU under a specific SA.  This function can be simplified once
we get rid all other uses of get_max_size as we can move the calculation
into the invidual SA type.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/xfrm.h

@@ -807,6 +807,7 @@ extern void xfrm_state_flush(u8 proto);
 extern int xfrm_replay_check(struct xfrm_state *x, u32 seq);
 extern void xfrm_replay_advance(struct xfrm_state *x, u32 seq);
 extern int xfrm_state_check(struct xfrm_state *x, struct sk_buff *skb);
+extern int xfrm_state_mtu(struct xfrm_state *x, int mtu);
 extern int xfrm4_rcv(struct sk_buff *skb);
 extern int xfrm4_output(struct sk_buff *skb);
 extern int xfrm4_tunnel_register(struct xfrm_tunnel *handler);

net/xfrm/xfrm_state.c

@@ -966,6 +966,36 @@ void xfrm_state_delete_tunnel(struct xfrm_state *x)
 }
 EXPORT_SYMBOL(xfrm_state_delete_tunnel);
 
+int xfrm_state_mtu(struct xfrm_state *x, int mtu)
+{
+	int res = mtu;
+
+	res -= x->props.header_len;
+
+	for (;;) {
+		int m = res;
+
+		if (m < 68)
+			return 68;
+
+		spin_lock_bh(&x->lock);
+		if (x->km.state == XFRM_STATE_VALID &&
+		    x->type && x->type->get_max_size)
+			m = x->type->get_max_size(x, m);
+		else
+			m += x->props.header_len;
+		spin_unlock_bh(&x->lock);
+
+		if (m <= mtu)
+			break;
+		res -= (m - mtu);
+	}
+
+	return res;
+}
+
+EXPORT_SYMBOL(xfrm_state_mtu);
+ 
 void __init xfrm_state_init(void)
 {
 	int i;