policy: replace RWMutex with Mutex
policy: replace RWMutex with Mutex

This is only locked using write locks, so replacing it with a normal
mutex would reduce the overhead by a tiny bit.

This could probably see a lot of improvements by rewriting to a proper
rwmutex and use read locking, but I think we should have some kind of
benchmark to ensure that change would be net positive; and I think
this commit is a positive step towards that.

Signed-off-by: Odin Ugedal <785541f0e54bdd28cf396d95bb334a566c03f369@palantir.com>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium