RDMA/mlx5: Use get_zeroed_page() for clock_info

get_zeroed_page() returns a virtual address for the page which is better
than allocating a struct page and doing a permanent kmap on it.

Cc: stable@vger.kernel.org
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>