gre_gso_segment() chokes if SIT frames were aggregated by GRO engine.
Fixes: 61c1db7fae21e ("ipv6: sit: add GSO/TSO support") Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>