IB/mlx5: Fix warning about cast of wr_id back to pointer on 32 bits

We need to cast wr_id to unsigned long before casting to a pointer.
This fixes:

       drivers/infiniband/hw/mlx5/mr.c: In function 'mlx5_umr_cq_handler':
    >> drivers/infiniband/hw/mlx5/mr.c:724:13: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
          context = (struct mlx5_ib_umr_context *)wc.wr_id;

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>