vm: fix sysctl vm.kstack_cache_size change report

Cache gets resized correctly, but sysctl reports the wrong number:

sysctl vm.kstack_cache_size=512

vm.kstack_cache_size: 128 -> 128

patched:
vm.kstack_cache_size: 128 -> 512

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D22717
Fixes: r355002 "Revise the page cache size policy."