IB/mlx5: Fix mlx5_ib_alloc_mr error flow

ibmr.device is being set only after ib_alloc_mr() is
(successfully) complete. Therefore, in case mlx5_core_create_mkey()
return with error, the error flow calls mlx5_free_priv_descs()
which uses ibmr.device (which doesn't exist yet), causing
a NULL dereference oops.

To fix this, the IB device should be set in the mr struct earlier
stage (e.g. prior to calling mlx5_core_create_mkey()).

Fixes: 8a187ee5

 ("IB/mlx5: Support the new memory registration API")
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Nitzan Carmi <nitzanc@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>