null_blk: return fixed zoned reads > write pointer

A zoned block device maintains a write pointer within a zone, and reads
beyond the write pointer are undefined. Fill data buffer returned above
the write pointer with 0xFF.

Signed-off-by: Ajay Joshi <ajay.joshi@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Matias Bjørling <matias.bjorling@wdc.com>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>