backup_calculate_cluster_size does not consider source

[Dietmar Maurer]

Example: Backup from ceph disk (rbd_cache=false) to local disk:

backup_calculate_cluster_size returns 64K (correct for my local .raw image)

Then the backup job starts to read 64K blocks from ceph.

But ceph always reads 4M block, so this is incredibly slow and produces
way too much network traffic.

Why does backup_calculate_cluster_size does not consider the block size from
the source disk? 

cluster_size = MAX(block_size_source, block_size_target)