arch_io_reserve_memtype_wc() persists 4 uninitialized stack bytes into the global memrange descriptor array (mr_owner[4..7] leak into MEMRANGE_GET)

Summary

Partial init + strlcpy('drm') leaves mr_owner[4..7] stale stack bytes that amd64_mrsetlow/variable bcopies into the persistent descriptor array - handed back via /dev/mem MEMRANGE_GET. Privileged-only disclosure (Info). memset-before-fill fix.