nbd: call blk_mark_disk_dead in nbd_clear_sock_ioctl

nbd_clear_sock_ioctl kills the socket and with that the block
device.  Instead of just invalidating file system buffers,
mark the device as dead, which will also invalidate the buffers
as part of the proper shutdown sequence.  This also includes
invalidating partitions if there are any.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Message-Id: <20230811100828.1897174-8-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>