SUNRPC: Ensure memory shrinker doesn't waste time in rpcauth_prune_expired()

The 'cred_unused' list, that is traversed by rpcauth_cache_shrinker is
ordered by time. If we hit a credential that is under the 60 second garbage
collection moratorium, we should exit because we know at that point that
all successive credentials are subject to the same moratorium...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>