r355002 "Revise the page cache size policy." broke reporting. I verified the cache gets resized correctly both before and after the patch. Handling of the parameter was stolen from sysctl_update_desiredvnodes.
sysctl vm.kstack_cache_size=512 before:
vm.kstack_cache_size: 128 -> 128
after:
vm.kstack_cache_size: 128 -> 512