cred: add prepare credential guard
A lot of code uses the following pattern:
* prepare new credentials
* modify them for their use-case
* drop them
Support that easier with the new guard infrastructure.
Link: https://patch.msgid.link/20251103-work-creds-guards-prepare_creds-v1-1-b447b82f2c9b@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>