RDMA/mlx5: Remove npages from mlx5_ib_cont_pages()

Most callers don't need this, and the few that do can get it as
ib_umem_num_pages(umem).

Link: https://lore.kernel.org/r/20201026131936.1335664-8-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>