x86/mm: avoid playing with directmap when self-snoop can be relied upon

x86/mm: avoid playing with directmap when self-snoop can be relied upon

The set of systems affected by XSA-345 would have been smaller is we had
this in place already: When the processor is capable of dealing with
mismatched cacheability, there's no extra work we need to carry out.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>