avoid healObjects recursively healing at empty path (#11856)

baseDirFromPrefix(prefix) for object names without
parent directory incorrectly uses empty path, leading
to long listing at various paths that are not useful
for healing - avoid this listing completely if "baseDir"
returns empty simple use the "prefix" as is.

this improves startup performance significantly

cmd/erasure-server-pool.go

@@ -1563,10 +1563,15 @@ func (z *erasureServerPools) HealObjects(ctx context.Context, bucket, prefix str
 						bucket:    bucket,
 					}
 
+					path := baseDirFromPrefix(prefix)
+					if path == "" {
+						path = prefix
+					}
+
 					if err := listPathRaw(ctx, listPathRawOptions{
 						disks:          disks,
 						bucket:         bucket,
-						path:           baseDirFromPrefix(prefix),
+						path:           path,
 						recursive:      true,
 						forwardTo:      "",
 						minDisks:       1,