libfs: massage path_from_stashed() to allow custom stashing behavior

libfs: massage path_from_stashed() to allow custom stashing behavior

* Add a callback to struct stashed_operations so it's possible to
  implement custom behavior for pidfs and allow for it to return errors.

* Teach stashed_dentry_get() to handle error pointers.

Link: https://lore.kernel.org/20250618-work-pidfs-persistent-v2-2-98f3456fd552@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>