{IB,net}/mlx5: Setup mkey variant before mr create command invocation

On reg_mr_callback() mlx5_ib is recalculating the mkey variant which is
wrong and will lead to using a different key variant than the one
submitted to firmware on create mkey command invocation.

To fix this, we store the mkey variant before invoking the firmware
command and use it later on completion (reg_mr_callback).

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>