IB/mlx5: Update the dev in reg_create

When we create an MR using reg_create, the mlx5_ib_dev pointer is not
updated on the new MR. This results in a kernel panics for ODP MRs
while handling page faults, when the mlx5_ib_update_mtt function uses
the invalid device pointer.

Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>