ovl: don't override credentials for ovl_check_whiteouts()

ovl: don't override credentials for ovl_check_whiteouts()

The function is only called when rdd->dentry is non-NULL:

if (!err && rdd->first_maybe_whiteout && rdd->dentry)
    err = ovl_check_whiteouts(realpath, rdd);

| Caller                        | Sets rdd->dentry? | Can call ovl_check_whiteouts()? |
|-------------------------------|-------------------|---------------------------------|
| ovl_dir_read_merged()         | ✓ Yes (line 430)  | ✓ YES                           |
| ovl_dir_read_impure()         | ✗ No              | ✗ NO                            |
| ovl_check_d_type_supported()  | ✗ No              | ✗ NO                            |
| ovl_workdir_cleanup_recurse() | ✗ No              | ✗ NO                            |
| ovl_indexdir_cleanup()        | ✗ No              | ✗ NO                            |

VFS layer (.iterate_shared file operation)
  → ovl_iterate()
      [CRED OVERRIDE]
      → ovl_cache_get()
          → ovl_dir_read_merged()
              → ovl_dir_read()
                  → ovl_check_whiteouts()
      [CRED REVERT]

ovl_unlink()
  → ovl_do_remove()
      → ovl_check_empty_dir()
          [CRED OVERRIDE]
          → ovl_dir_read_merged()
              → ovl_dir_read()
                  → ovl_check_whiteouts()
          [CRED REVERT]

ovl_rename()
  → ovl_check_empty_dir()
      [CRED OVERRIDE]
      → ovl_dir_read_merged()
          → ovl_dir_read()
              → ovl_check_whiteouts()
      [CRED REVERT]

All valid callchains already override credentials so drop the override.

Link: https://patch.msgid.link/20251117-work-ovl-cred-guard-v4-24-b31603935724@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>