block/rnbd-srv: close a mapped device from server side.

The forceful close of an exported device is required
for the use case, when the client side hangs, is crashed,
or is not accessible.

There have been cases observed, where only some of
the devices are to be cleaned up, but the session shall
remain.

When the device is to be exported to a different
client host, server side cleanup is required.

Signed-off-by: Lutz Pogrell <lutz.pogrell@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Reviewed-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>