Basic smoketest in a VM with 'Svpbmt' extension enabled: this only has a couple mappings marked "I/O" in `sysctl vm.pmap.kernel_maps`.
`pmap_change_attr()` is not widely used, so to test this I swapped some driver `malloc()` calls to allocate memory/KVA but map it as `VM_MEMATTR_NOCACHE`. This seemed to work fine and the DMAP was updated accordingly, forcing a demotion of the L1 page created by `pmap_bootstrap_dmap()`.