block: freeze the queue earlier in del_gendisk

Freeze the queue earlier in del_gendisk so that the state does not
change while we remove debugfs and sysfs files.

Ming mentioned that being able to observer request in debugfs might
be useful while the queue is being frozen in del_gendisk, which is
made possible by this change.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220614074827.458955-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>