[$] Faster reference-count overflow protection

Improving the security of a system often involves tradeoffs, with the costsmeasured in terms of convenience and performance, among others. To theirfrustration, security-oriented developers often discover that the tolerancefor these costs is quite low. Defenses against reference-count overflowshave run into that sort of barrier, slowing their adoption considerably.Now, though, it would appear that a solution has been found to theperformance cost imposed by reference-count hardening, clearing the waytoward its adoption throughout the kernel.