The block debugfs files are created in blk_register_queue, which is
called by add_disk and use a naming scheme based on the disk_name.
After del_gendisk returns that name can be reused and thus we must not
leave these debugfs files around, otherwise the kernel is unhappy
and spews messages like:

	Directory XXXXX with parent 'block' already present!

and the newly created devices will not have working debugfs files.

Move the unregistration to blk_unregister_queue instead (which matches
the sysfs unregistration) to make sure the debugfs life time rules match
those of the disk name.

As part of the move also make sure the whole debugfs unregistration is
inside a single debugfs_mutex critical section.

Note that this breaks blktests block/002, which checks that the debugfs
directory has not been removed while blktests is running, but that
particular check should simply be removed from the test case.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220614074827.458955-4-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>