mlx4: get rid of frag_prefix_size

Using per frag storage for frag_prefix_size is really silly.

mlx4_en_complete_rx_desc() has all needed info already.

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