IB/mlx5: Set network_hdr_type upon RoCE responder completion

When handling a responder completion, if the link layer is Ethernet,
set the work completion network_hdr_type field according to CQE's
info and the IB_WC_WITH_NETWORK_HDR_TYPE flag.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>