fix(master): stop endless volume growth in DCs with more racks than replica count (#8986)
ShouldGrowVolumesByDcAndRack checked every DC+rack for a writable volume
replica. With "010" replication (different-rack), volumes only span 2 racks.
In a DC with 3+ racks, at least one rack always lacked a replica, causing
the periodic growth loop to create new volumes endlessly.
When DiffRackCount > 0, check at the DC level instead: if any rack in the
DC has a non-crowded writable volume, skip growth for uncovered racks.
Chris LuandGitHub
8b16507059