net/mlx4_en: CHECKSUM_COMPLETE support for fragments

Refine the RX check summing handling to propagate the
hardware provided checksum so that we do not have to
compute it later in software.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Acked-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>