mlx4: add page recycling in receive path

Same technique than some Intel drivers, for arches where PAGE_SIZE = 4096

In most cases, pages are reused because they were consumed
before we could loop around the RX ring.

This brings back performance, and is even better,
a single TCP flow reaches 30Gbit on my hosts.

v2: added full memset() in mlx4_en_free_frag(), as Tariq found it was needed
if we switch to large MTU, as priv->log_rx_info can dynamically be changed.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/mlx4_en.h

@@ -327,7 +327,6 @@ struct mlx4_en_rx_desc {
 
 struct mlx4_en_rx_ring {
 	struct mlx4_hwq_resources wqres;
-	struct mlx4_en_rx_alloc page_alloc[MLX4_EN_MAX_RX_FRAGS];
 	u32 size ;	/* number of Rx descs*/
 	u32 actual_size;
 	u32 size_mask;