ceph-volume: fix fast device alloc size on mulitple device

ceph-volume: fix fast device alloc size on mulitple device

The size computed by get_physical_fast_allocs() was wrong when the
function had multiple devices to treat.

For instance if there is 4 OSDs and 2 fast devices of each 10G while
allocating 2 slots per fast devvices. The behavior before was that each
slot would be 2.5G meaning that both fast devices would half full. The
behavior now is that each slot will take 5G so that the fast devices
would be full.

Fixes: https://tracker.ceph.com/issues/56031
Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@cern.ch>
(cherry picked from commit d0f9e93914e2b7feac41a634311d74c146c8868b)