Make the direct-map API always available, but fail if KASAN or rump.

(Only for architectures that support it at all; on others,
__HAVE_MM_MD_DIRECT_MAPPED_PHYS/IO are still undefined and the
functions unimplemented.)

This gives modules like zfs an opportunity to use it.

While here, fix the one caller of mm_md_direct_mapped_phys that
ignored the return value (and make sure to call pmap_kremove/update
before uvm_km_free).

(riastradh)