tar: fix numbered backup bug

* src/misc.c (maybe_backup_file): Pass chdir_fd, not AT_FDCWD,
to find_backup_file_name.  This fixes a bug whereby the wrong
backup file name was chosen for numbered backups.

src/misc.c

@@ -727,7 +727,7 @@ maybe_backup_file (const char *file_name, bool this_is_the_archive)
       && (S_ISBLK (file_stat.st_mode) || S_ISCHR (file_stat.st_mode)))
     return true;
 
-  after_backup_name = find_backup_file_name (AT_FDCWD, file_name, backup_type);
+  after_backup_name = find_backup_file_name (chdir_fd, file_name, backup_type);
   if (! after_backup_name)
     xalloc_die ();