RDMA/mlx5: Move mlx5_ib_cont_pages() to the creation of the mlx5_ib_mr

For the user MR path, instead of calling this after getting the umem, call
it as part of creating the struct mlx5_ib_mr and distill its output to a
single page_shift stored inside the mr.

This avoids passing around the tuple of its output. Based on the umem and
page_shift, the output arguments can be computed using:

  count == ib_umem_num_pages(mr->umem)
  shift == mr->page_shift
  ncont == ib_umem_num_dma_blocks(mr->umem, 1 << mr->page_shift)
  order == order_base_2(ncont)

And since mr->page_shift == umem_odp->page_shift then ncont ==
ib_umem_num_dma_blocks() == ib_umem_odp_num_pages() for ODP umems.

Link: https://lore.kernel.org/r/20201026131936.1335664-5-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>