x86/mm: avoid inadvertently degrading a TLB flush to local only

x86/mm: avoid inadvertently degrading a TLB flush to local only

If the direct map is incorrectly modified with interrupts disabled,
the required TLB flushes are degraded to flushing the local CPU only.

This could lead to very hard to diagnose problems as different CPUs will
end up with different views of memory. Although, no such issues have yet
been identified.

Change the check in the flush_area() macro to look at system_state
instead. This defers the switch from local to all later in the boot
(see xen/arch/x86/setup.c:__start_xen()). This is fine because
additional PCPUs are not brought up until after the system state is
SYS_STATE_smp_boot.

Signed-off-by: David Vrabel <dvrabel@amazon.co.uk>
Reviewed-by: Jan Beulich <jbeulich@suse.com>