sev-dev: use guard for path
Just use a guard and also move the path_put() out of the credential
change's scope. There's no need to do this with the overridden
credentials.
Link: https://patch.msgid.link/20251103-work-creds-guards-prepare_creds-v1-2-b447b82f2c9b@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>