IB/mlx5: Fix integer overflow when page_shift == 31

Fix a bug where MR registration fails when mlx5_ib_cont_pages
indicates that the MR can be mapped using 2GB pages (page_shift == 31).

Fixes: e126ba97 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Signed-off-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>