null_blk: fix zone size paramter check

For zoned=1 mode, the zone size must be a power of 2. Check this not
only when the zone size is specified during modprobe, but also when
creating a zoned null_blk device using configfs.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>