winbindd_cache: simplify logic in new key length check for UA keys.

winbindd_cache: simplify logic in new key length check for UA keys.

This reduces indentation by combining common code paths,
and wraps long lines.

Holger: sorry, I could not resist. I think it is much easier to
understand what is going on when we only have one check and
determine the max allowed key length in advance.

Michael
(This used to be commit e489f3d988feafe35b486b31a9e60c2399e6a6e7)