net/mlx4_en: Rename name of mlx4_en_rx_alloc members

Add page prefix to page related members: @size and @offset into
@page_size and @page_offset

CC: Eric Dumazet <edumazet@google.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>