IB/core: Set correct payload length for RoCEv2 over IPv6

For GSI QP traffic, the count of the udp header bytes was missing from
the IPv6 header, fix that.

Fixes: 25f40220 ('IB/core: Initialize UD header structure with IP
                     and UDP headers')
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>