amd64: switch pmap_map_io_transient() to use pmap_kenter_attr()

instead of constructing transient pte itself. This pre-set PG_A and
PG_M bits, avoiding atomic pte update on access and modification. Also
it set the nx bit, the mapping is not supposed to be used for executing.

Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D47717