block/rnbd-srv: Protect dev session sysfs removal

Since the removal of the session sysfs can also be called from the
function destroy_sess, there is a need to protect the call from the
function rnbd_srv_sess_dev_force_close

Fixes: 78699805

 ("block/rnbd-srv: close a mapped device from server side.")
Signed-off-by: Md Haris Iqbal <haris.iqbal@cloud.ionos.com>
Reviewed-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>