x86/speculation/l1tf: Protect NUMA-balance entries against L1TF

x86/speculation/l1tf: Protect NUMA-balance entries against L1TF

NUMA balancing has its own functions that manipulated the PRESENT flag
in PTEs and PMDs.  These were not affected by the changes in commit
6b28baca9b1f "x86/speculation/l1tf: Protect PROT_NONE PTEs against
speculation".

This is not a problem upstream because NUMA balancing was changed to
use {pte,pmd}_modify() in Linux 4.0.

Override the generic implementations for x86 with implementations
that do the same inversion as {pte,pmd}_modify().

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: x86@kernel.org
Cc: Mel Gorman <mgorman@suse.de>