fs: disentagle dry-run ntfsresize loop to probe for best shrink size

Revisit 5056b8f0d5 ("fs: validate ntfsresize dry-run output") that has
introduced a possible infinity loop.

Disentangle this loop while at it: iterate until best smallest size is
found by probing.

src/utils/fs.py

@@ -235,19 +235,27 @@ def _reduce_ntfsresize(partdev):
     size = int(out_info.split('device size: ')[1].split(' ')[0])
     new_size = int(int(out_info.split('resize at ')[1].split(' ')[0])*1.1+1024)
 
-    # Dry-run loop to test if resizing is actually possible. This is required by ntfsresize.
-    returncode = 1
-    while new_size < size and returncode != 0:
+    # Run ntfsresize with -n to to probe for the smallest size, this loop is
+    # intentional. Acumulate size until ntfsresize in dry-run mode fails, then
+    # use such size.
+    while new_size < size:
         cmd_resize_dryrun = shlex.split(f'ntfsresize -Pfns {new_size:.0f} {partdev}')
         proc_resize_dryrun = subprocess.run(cmd_resize_dryrun, stdout=subprocess.PIPE, encoding='utf-8')
-        returncode = proc_resize_dryrun.returncode
+
+        # valid new size found, stop probing
+        if proc_resize_dryrun.returncode == 0:
+            break
+
         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?')
+            logging.info('ntfsresize reports nothing to do. Is the target filesystem already shrunken?')
             break
-        if out_resize_dryrun.find('Needed relocations : ') != -1:
-            extra_size = int(out_resize_dryrun.split('Needed relocations : ')[1].split(' ')[0])*1.1+1024
-            new_size += int(extra_size)
+
+        if out_resize_dryrun.find('Needed relocations : ') == -1:
+            break
+
+        extra_size = int(out_resize_dryrun.split('Needed relocations : ')[1].split(' ')[0])*1.1+1024
+        new_size += int(extra_size)
 
     if new_size < size:
         cmd_resize = shlex.split(f'ntfsresize -fs {new_size:.0f} {partdev}')