zbd: fix write zone accounting of almost full zones

zbd: fix write zone accounting of almost full zones

For zonemode=zbd, fio checks condition of each zone and account it as
write target zone if it has open condition. However, when such a zone in
open condition is almost full and its remainder area for write is
smaller than the block size, fio does not handle it as a write target
zone. This causes difference between open zones accounting on the device
and write target zones accounting by fio. It results in unexpected
max_open_zones limit failure.

Avoid the zone accounting difference by handling the almost full zones
as write target zones at fio start. Introduce the helper function
__zbd_write_zone_get() which does same operation as zbd_write_zone_get()
except the check for the almost full zones. At fio start, call
__zbd_write_zone_get() so that almost full zones are added to write
target zones. During fio workload run, call zbd_write_zone_get() so that
the almost full zones are not chosen for write target.

Suggested-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>