KVM: x86: Unify L1TF flushing under per-CPU variable

Currently the tracking of the need to flush L1D for L1TF is tracked by
two bits: one per-CPU and one per-vCPU.

The per-vCPU bit is always set when the vCPU shows up on a core, so
there is no interesting state that's truly per-vCPU. Indeed, this is a
requirement, since L1D is a part of the physical CPU.

So simplify this by combining the two bits.

The vCPU bit was being written from preemption-enabled regions.  To play
nice with those cases, wrap all calls from KVM and use a raw write so that
request a flush with preemption enabled doesn't trigger what would
effectively be DEBUG_PREEMPT false positives.  Preemption doesn't need to
be disabled, as kvm_arch_vcpu_load() will mark the new CPU as needing a
flush if the vCPU task is migrated, or if userspace runs the vCPU on a
different task.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
[sean: put raw write in KVM instead of in a hardirq.h variant]
Link: https://patch.msgid.link/20251113233746.1703361-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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