mlx4: add rx_alloc_pages counter in ethtool -S

This new counter tracks number of pages that we allocated for one port.

lpaa24:~# ethtool -S eth0 | egrep 'rx_alloc_pages|rx_packets'
     rx_packets: 306755183
     rx_alloc_pages: 932897

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