vdpa/mlx5: Use readers/writers semaphore instead of mutex

Reading statistics could be done intensively and by several processes
concurrently. Reader's lock is sufficient in this case.

Change reslock from mutex to a rwsem.

Suggested-by: Si-Wei Liu <si-wei.liu@oracle.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20220518133804.1075129-7-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>