zbd: factor out zbd_pick_write_zone()

zbd: factor out zbd_pick_write_zone()

To prepare for the following fix, factor out a part of
zbd_convert_to_write_zone() to the new function zbd_pick_write_zone().
This function randomly chooses a zone in the array of write zones.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250414062721.87641-3-shinichiro.kawasaki@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>