null_blk: Fix zoned command return code

The return code from null_handle_zoned() sets the cmd->error value.
Returning OK status when an error occured overwrites the intended
cmd->error. Return the appropriate error code instead of setting the
error in the cmd.

Fixes: fceb5d1b

 ("null_blk: create a helper for zoned devices")
Cc: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>