zbd: rename zbd_zone_idx() and zbd_zone_nr()

zbd: rename zbd_zone_idx() and zbd_zone_nr()

Rename zbd_zone_idx() to zbd_offset_to_zone_idx() to make it clear that
the argument determining the zone is a file offset. To be consistent,
rename zbd_zone_nr() to zbd_zone_idx() to avoid confusion with a number
of zones. While at it, have both functions return value be of the same
unsigned int type.

No functional changes.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20211214012413.464798-10-damien.lemoal@opensource.wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>