zbd: change some f->zbd_info conditionals to asserts

zbd: change some f->zbd_info conditionals to asserts

Unfortunately, generic fio code calls some zbd_* functions unconditionally.
These functions will be called regardless if zonemode == ZONE_MODE_NONE,
ZONE_MODE_STRIDED or ZONE_MODE_ZBD, and cannot be optimized.

However, some functions are only called when zonemode == ZONE_MODE_ZBD.
Since f->zbd_info will always be non-NULL for a job with zonemode=zbd,
these functions can be optimized to not check if f->zbd_info is set.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>