Merge patch series "ovl: convert copyup credential override to cred guard"

Christian Brauner <brauner@kernel.org> says:

This simplifies the copyup specific credential override.

The current code is centered around a helper struct ovl_cu_creds and is
a bit convoluted. We can simplify this by using a cred guard. This will
also allow us to remove the helper struct and associated functions.

* patches from https://patch.msgid.link/20251114-work-ovl-cred-guard-copyup-v1-0-ea3fb15cf427@kernel.org:
  ovl: remove struct ovl_cu_creds and associated functions
  ovl: port ovl_copy_up_tmpfile() to cred guard
  ovl: mark *_cu_creds() as unused temporarily
  ovl: port ovl_copy_up_workdir() to cred guard
  ovl: add copy up credential guard

Link: https://patch.msgid.link/20251114-work-ovl-cred-guard-copyup-v1-0-ea3fb15cf427@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>