null_blk: don't allow discard for zoned mode

Zoned block device specification do not define the behavior of
discard/trim command as this command is generally replaced by the reset
write pointer (zone reset) command. Emulate this in null_blk by making
zoned and discard options mutually exclusive.

Suggested-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>