cachefiles: avoid pointless cred reference count bump

cachefiles: avoid pointless cred reference count bump

The cache holds a long-term reference to the credentials that's taken
when the cache is created and put when the cache becomes unused.

Link: https://lore.kernel.org/r/20241125-work-cred-v2-27-68b9d38bb5b2@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>