block: don't set GENHD_FL_NO_PART for hidden gendisks

Hidden gendisks can't be opened using blkdev_get_*, so we can't really
reach any of the partition scanning paths or partitioning ioctls except
for the initial partition scan from add_disk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211122130625.1136848-13-hch@lst.de


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