Add more erasure codes on degraded systems. (#11852)

In cases where a cluster is degraded, we do not uphold our consistency 
guarantee and we will write fewer erasure codes and rely on healing 
to recreate the missing shards.

In some cases replacing known bad disks in practice take days.
We want to change the behavior of a known degraded system to keep
the erasure code promise of the storage class for each object.

This will create the objects with the same confidence as a fully 
functional cluster. The tradeoff will be that objects created 
during a partial outage will take up slightly more space.

This means that when the storage class is EC:4, there should 
always be written 4 parity shards, even if some disks are unavailable.

When an object is created on a set, the disks are immediately 
checked. If any disks are unavailable additional parity shards 
will be made for each offline disk, up to 50% of the number of disks.

We add an internal metadata field with the actual and intended 
erasure code level, this can optionally be picked up later by 
the scanner if we decide that data like this should be re-sharded.

cmd/erasure-object.go

@@ -599,6 +599,24 @@ func (er erasureObjects) putObject(ctx context.Context, bucket string, object st
 		if parityDrives <= 0 {
 			parityDrives = er.defaultParityCount
 		}
+
+		// If we have offline disks upgrade the number of erasure codes for this object.
+		ecOrg := parityDrives
+		for _, disk := range storageDisks {
+			if parityDrives >= len(storageDisks)/2 {
+				break
+			}
+			if disk == nil {
+				parityDrives++
+			}
+			di, err := disk.DiskInfo(ctx)
+			if err != nil || di.ID == "" {
+				parityDrives++
+			}
+		}
+		if ecOrg != parityDrives {
+			opts.UserDefined[xhttp.MinIOErasureUpgraded] = fmt.Sprintf("%d->%d", ecOrg, parityDrives)
+		}
 	}
 	dataDrives := len(storageDisks) - parityDrives