ovl: allow to specify override credentials

Currently overlayfs uses the mounter's credentials for it's
override_creds() calls. That provides a consistent permission model.

This patches allows a caller to instruct overlayfs to use its
credentials instead. The caller must be located in the same user
namespace hierarchy as the user namespace the overlayfs instance will be
mounted in. This provides a consistent and simple security model.

With this it is possible to e.g., mount an overlayfs instance where the
mounter must have CAP_SYS_ADMIN but the credentials used for
override_creds() have dropped CAP_SYS_ADMIN. It also allows the usage of
custom fs{g,u}id different from the callers and other tweaks.

Link: https://lore.kernel.org/r/20250219-work-overlayfs-v3-1-46af55e4ceda@kernel.org


Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>