block/genhd: port disk_scan_partitions() to file

This may run from a kernel thread via device_add_disk(). So this could
also use __fput_sync() if we were worried about EBUSY. But when it is
called from a kernel thread it's always BLK_OPEN_READ so EBUSY can't
really happen even if we do BLK_OPEN_RESTRICT_WRITES or BLK_OPEN_EXCL.

Otherwise it's called from an ioctl on the block device which is only
called from userspace and can rely on task work.

Link: https://lore.kernel.org/r/20240123-vfs-bdev-file-v2-3-adbd023e19cc@kernel.org


Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>