xen-blkfront: sanitize the removal state machine

xen-blkfront has a weird protocol where close message from the remote
side can be delayed, and where hot removals are treated somewhat
differently from regular removals, all leading to potential NULL
pointer removals, and a del_gendisk from the block device release
method, which will deadlock. Fix this by just performing normal hot
removals even when the device is opened like all other Linux block
drivers.

Fixes: c76f48eb

 ("block: take bd_mutex around delete_partitions in del_gendisk")
Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20210715141711.1257293-1-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>