fs: fix bug when ntfsresize reports nothing to do

There is a corner case in which a target NTFS filesystem is already
shrunken. When this happens ntfsresize text output parsing breaks.

Check when ntfsresize reports nothing to do, warn the user about this
and stop the dry-run ntfsresize loop.

src/utils/fs.py

@@ -229,6 +229,9 @@ def _reduce_ntfsresize(partdev):
         proc_resize_dryrun = subprocess.run(cmd_resize_dryrun, stdout=subprocess.PIPE, encoding='utf-8')
         returncode = proc_resize_dryrun.returncode
         out_resize_dryrun = proc_resize_dryrun.stdout.strip()
+        if 'Nothing to do: NTFS volume size is already OK.' in out_resize_dryrun:
+            logging.warn('ntfsresize reports nothing to do. Is the target filesystem already shrunken?')
+            break
         extra_size = int(out_resize_dryrun.split('Needed relocations : ')[1].split(' ')[0])*1.1+1024
         new_size += int(extra_size)