[XFRM]: Fix MTU calculation for non-ESP SAs

My IPsec MTU optimization patch introduced a regression in MTU calculation
for non-ESP SAs, the SA's header_len needs to be subtracted from the MTU if
the transform doesn't provide a ->get_mtu() function.

Reported-and-tested-by: Marco Berizzi <pupilla@hotmail.com>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 1 deletions

diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index 85f3f43a6cca..dfacb9c2a6e3 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -1729,7 +1729,7 @@ int xfrm_state_mtu(struct xfrm_state *x, int mtu)
             x->type && x->type->get_mtu)
                 res = x->type->get_mtu(x, mtu);
         else
-                res = mtu;
+                res = mtu - x->props.header_len;
         spin_unlock_bh(&x->lock);
         return res;
 }