block: warn when putting the final reference on a registered disk

Warn when the last reference on a live disk is put without calling
del_gendisk first.  There are some BDI related bug reports that look
like a case of this, so make sure we have the proper instrumentation
to catch it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211014130231.1468538-1-hch@lst.de


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