xfrm6: Fix IPv6 payload_len in xfrm6_transport_finish

IPv6 payload length indicates the size of the payload, including any
extension headers.

In xfrm6_transport_finish, ipv6_hdr(skb)->payload_len is set to the
payload size only, regardless of the presence of any extension headers.
After ESP GRO transport mode decapsulation, ipv6_rcv trims the packet
according to the wrong payload_len, thus corrupting the packet.

Set payload_len to account for extension headers as well.

Fixes: 7785bba2 ("esp: Add a software GRO codepath")
Signed-off-by: Yossi Kuperman <yossiku@mellanox.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>