net/mlx5: Reset mkey index on creation

Reset only the index part of the mkey and keep the variant part. On
devlink reload, driver recreates mkeys, so the mkey index may change.
Trying to preserve the variant part of the mkey, driver mistakenly
merged the mkey index with current value. In case of a devlink reload,
current value of index part is dirty, so the index may be corrupted.

Fixes: 54c62e13 ("{IB,net}/mlx5: Setup mkey variant before mr create command invocation")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Signed-off-by: Amir Tzin <amirtz@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>