xen-blkfront: Fix IO race during unplug

During unplug, blkfront stops the hw queues and marks the disk as dead,
then later during removal calls del_gendisk(). However, IO issued after
the hw queues are stopped but before the call to del_gendisk() will be
queued but never handled. This causes del_gendisk() to hang forever
waiting for the queue refcount to drop to zero.

This can be reproduced by issuing IO during an artificial delay after
stopping the hw queues.

Fix this by simply not stopping the hw queues directly. Marking the disk
as dead already freezes the queue which prevents new requests being
added and it synchronously runs the hw queues to clear anything pending.
If the hw queues are stopped when calling blk_mark_disk_dead(), running
the hw queues is a no-op and can leave queued requests unprocessed.

Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Roger Pau Monné <roger@xenproject.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20261006163411.2583538-1-ross.lagerwall@citrix.com>