net/vdpa: Use readers/writers semaphore instead of vdpa_dev_mutex

Use rw_semaphore instead of mutex to control access to vdpa devices.
This can be especially beneficial in case processes poll on statistics
information.

Suggested-by: Si-Wei Liu <si-wei.liu@oracle.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20220518133804.1075129-4-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>