fs/smartfs: Fix file size corruption when opening with truncate mode

If a existing file is opened with truncate mode e.g. fopen(file, "w+"),
the file size will be incorrect after writing any data to the file.
Before writing to the first sector, the reading is performed again.
As a result, it makes an invalid file size. When a sector buffer is used,
it must also write to the first sector.

fs/smartfs/smartfs_utils.c

@@ -1765,8 +1765,8 @@ int smartfs_shrinkfile(FAR struct smartfs_mountpt_s *fs,
               remaining = 0;
             }
         }
-      else
 #endif
+
       /* Are we retaining the sector it its entirety? */
 
       if (remaining >= available)