Uninitialized kernel stack memory disclosed via vm.resident sysctl

Summary

sysctl_vm_resident: struct xresident xres on stack uninitialized(:148). fill_xresident: struct st uninitialized(:82). On vget/vn_stat failure paths(:103-105,:108-111 goto done), res_stat never written. SYSCTL_OUT(:153) copies entire struct including uninitialized stack -> kernel addr/pointer leak. Root-only (SYSCAP_NOVM_RESIDENT gate).