zbd: use zbd_zone_nr() more actively in the code

zbd: use zbd_zone_nr() more actively in the code

The function zbd_zone_nr() is always called with the first argument
being f->zbd_info. If "f" is made the first argument instead, calls
of this function become more compact end easier to read.

Besides this change, convert several places in the code where the same
zone number calculation is open coded to zbd_zone_nr() calls.
This is a refactoring patch, 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>