Validate gso_type during segmentation as SKB_GSO_DODGY sources
may pass packets where the gso_type does not match the contents.

Syzkaller was able to enter the SCTP gso handler with a packet of
gso_type SKB_GSO_TCPV4.

On entry of transport layer gso handlers, verify that the gso_type
matches the transport protocol.

Fixes: 90017acc ("sctp: Add GSO support")
Link: http://lkml.kernel.org/r/<001a1137452496ffc305617e5fe0@google.com>
Reported-by: syzbot+fee64147a25aecd48055@syzkaller.appspotmail.com
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Reviewed-by: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>