fix: leaking offline disks in MarkOffline() thread (#18414)

`monitorAndConnectEndpoints` will continue to attempt to reconnect offline disks.

Since disks were never closed, a `MarkOffline` would continue to try to check these disks forever.

Close previous disks.

cmd/erasure-sets.go

@@ -217,6 +217,10 @@ func (s *erasureSets) connectDisks() {
 				continue
 			}
 		}
+		if cdisk != nil {
+			// Close previous offline disk.
+			cdisk.Close()
+		}
 
 		wg.Add(1)
 		go func(endpoint Endpoint) {