genhd: Fix leaked module reference for NVME devices

Commit 8ddcd653 "block: introduce GENHD_FL_HIDDEN" added handling of
hidden devices to get_gendisk() but forgot to drop module reference
which is also acquired by get_disk(). Drop the reference as necessary.

Arguably the function naming here is misleading as put_disk() is *not*
the counterpart of get_disk() but let's fix that in the follow up
commit since that will be more intrusive.

Fixes: 8ddcd653
CC: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -817,7 +817,10 @@ struct gendisk *get_gendisk(dev_t devt, int *partno)
 	}
 
 	if (disk && unlikely(disk->flags & GENHD_FL_HIDDEN)) {
+		struct module *owner = disk->fops->owner;
+
 		put_disk(disk);
+		module_put(owner);
 		disk = NULL;
 	}
 	return disk;