zbd: move zone finish operation to zbd_convert_to_write_zone()

zbd: move zone finish operation to zbd_convert_to_write_zone()

Currently, when a write target zone has fewer remainder sectors than
the block size, fio finishes the zone to make the zone inactive (not
open), so that another zone can be open and used as a write target zone.
This zone finish operation is implemented in zbd_adjust_block().
However, this placement is less ideal because zbd_adjust_block() manages
not just write requests but also read and trim requests.

Since the zone finish operation is exclusively necessary for write
requests, implement it into zbd_convert_to_write_zone(). While at it,
improve the function comment.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250414062721.87641-2-shinichiro.kawasaki@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>