block: cleanup the GENHD_FL_* definitions

Switch to an enum and tidy up the documentation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211122130625.1136848-14-hch@lst.de


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