null_blk: Fail zone append to conventional zones

Conventional zones do not have a write pointer and so cannot accept zone
append writes. Make sure to fail any zone append write command issued to
a conventional zone.

Reported-by: Naohiro Aota <naohiro.aota@wdc.com>
Fixes: e0489ed5 ("null_blk: Support REQ_OP_ZONE_APPEND")
Cc: stable@vger.kernel.org
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>