ipv6: gre: support SIT encapsulation

gre_gso_segment() chokes if SIT frames were aggregated by GRO engine.

Fixes: 61c1db7f ("ipv6: sit: add GSO/TSO support")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>