net/mlx4_en: call gro handler for encapsulated frames

In order to use the native GRO handling of encapsulated protocols on
mlx4, we need to call napi_gro_receive() instead of netif_receive_skb()
unless busy polling is in action.

While we are at it, rename mlx4_en_cq_ll_polling() to
mlx4_en_cq_busy_polling()

Tested with GRE tunnel : GRO aggregation is now performed on the
ethernet device instead of being done later on gre device.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Amir Vadai <amirv@mellanox.com>
Cc: Jerry Chu <hkchu@google.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Acked-By: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>