zbd: add get_zone() helper function

zbd: add get_zone() helper function

The following pattern is used very widely in zbd.c -

zone = &f->zbd_info->zone_info[zone_idx] .

For the sake of code clarity, wrap this construct into an inline
helper. No change in functionality.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>