RDMA/mlx5: Remove mlx5_ib_mr->npages

This is the same value as ib_umem_num_pages(mr->umem), use that instead.

Link: https://lore.kernel.org/r/20201026131936.1335664-4-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>