zbd: finish zone when all random write target zones have small remainder

zbd: finish zone when all random write target zones have small remainder

When a random write target offset points to a zone that is not writable,
zbd_convert_to_write_zone() attempts to convert the write offset to a
different, writable zone. However, the conversion fails when all of the
following conditions are met:

1) the workload has the max_open_zones limit
2) every write target zones, up to the max_open_zones limit, has
   remainder sectors smaller than the block size
3) the next random write request targets a zone not in the write target
   zone list

In this case, zbd_convert_to_write_zone() can not open another zone
without exceeding the max_open_zones constraint. Therefore, It does not
convert the write to a different zone printing with the debug message
"did not choose another write zone". This leads to an unexpected stop of
the random write workload.

To prevent the unexpected write stop, finish one of the write target
zones with small remainder sectors. Check if all write target zones have
small remainder, and store the result in the new local boolean variable
all_write_zones_have_small_remainder. When this condition is true,
choose one of the write target zones and finish it. Then return the zone
from zbd_convert_to_write_zone() enabling the write process to continue.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250414062721.87641-4-shinichiro.kawasaki@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>