Fix several issues with /dev/mem and /dev/kmem devices on amd64.

For /dev/mem, when requested physical address is not accessible by the
direct map, do temporal remaping with the caching attribute
'uncached'. Limit the accessible addresses by MAXPHYADDR, since the
architecture disallowes writing non-zero into reserved bits of ptes
(or setting garbage into NX).

For /dev/kmem, only access existing kernel mappings for direct map
region. For all other addresses, obtain a physical address of the
mapping and fall back to the /dev/mem mechanism. This ensures that
/dev/kmem i/o does not fault even if the accessed region is changed in
parallel, by using either direct map or temporal mapping.

For both devices, operate on one page by iteration. Do not return
error if any bytes were moved around, return the (partial) bytes count
to userspace.

Reviewed by: alc
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week