net/mlx5_core: Use coherent memory for command interface page

Use coherent memory for the commands descriptor page. Take measures to make
sure the page is aligned to MLX5_ADAPTER_PAGE_SIZE as required by the hardware.

Reported-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>