posix: return errFaultyDisk on I/O errors. (#1885)

When I/O error is occured more than allowed limit, posix returns
errFaultyDisk.

Fixes #1884

tree-walk-xl.go

@@ -44,7 +44,7 @@ func (xl xlObjects) listDir(bucket, prefixDir string, filter func(entry string)
 		if err != nil {
 			// For any reason disk was deleted or goes offline, continue
 			// and list form other disks if possible.
-			if err == errDiskNotFound {
+			if err == errDiskNotFound || err == errFaultyDisk {
 				continue
 			}
 			break