null_blk: Allow controlling max_hw_sectors limit

Add the module option and configfs attribute max_sectors to allow
configuring the maximum size of a command issued to a null_blk device.
This allows exercising the block layer BIO splitting with different
limits than the default BLK_SAFE_MAX_SECTORS. This is also useful for
testing the zone append write path of file systems as the max_hw_sectors
limit value is also used for the max_zone_append_sectors limit.

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