ceph-volume: use correct extents when using db-devices and >1 osds_per_device

ceph-volume: use correct extents when using db-devices and >1 osds_per_device

Actual data size depending on osds_per_device needs to be calculated here. Otherwise, if osds_per_device is greater than 1, ceph-volume will allocate 100% of the device to the first osd and then fail to create the LV for the second because the volume group is already full.

Fixes: https://tracker.ceph.com/issues/39442
Signed-off-by: Fabian Niepelt <f.niepelt@mittwald.de>