null_blk: fixup ->report_zones() for !CONFIG_BLK_DEV_ZONED

A previous commit changed the prototype, but didn't adjust the function
for when zoned device support is disabled. Fix it up.

Fixes: bd976e52

 ("block: Kill gfp_t argument of blkdev_report_zones()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>