vdpa/mlx5: Move mr mutex out of mr struct

vdpa/mlx5: Move mr mutex out of mr struct

The mutex is named like it is supposed to protect only the mkey but in
reality it is a global lock for all mr resources.

Shift the mutex to it's rightful location (struct mlx5_vdpa_dev) and
give it a more appropriate name.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20231018171456.1624030-13-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Tested-by: Si-Wei Liu <si-wei.liu@oracle.com>
Tested-by: Lei Yang <leiyang@redhat.com>