ipv6: gre: support SIT encapsulation

ipv6: gre: support SIT encapsulation

Orabug: 22623835

[ Upstream commit 7e3b6e7423d5f994257c1de88e06b509673fdbcf ]

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>
(cherry picked from commit 199bcc1dc5bd32e70e104a42c56857c1062714f7)
Signed-off-by: Dan Duval <dan.duval@oracle.com>