KVM: SVM: Trigger new ASID allocation in both VMCBs on pCPU switch

When the pCPU where the VMCB was most recently used is switched, reset
the ASID generation in both VMCBs, triggering new ASID allocation for
the immediate VMRUN as well as the next VMRUN on the other VMCB.

The ASID is shared between vmcb01 and vmcb02, and gets flushed on every
nested transition. However, since pCPU tracking is done per VMCB, it is
possible for one VMCB to allocate a new ASID when migrated to a new
pCPU, and then the other VMCB reuses that ASID on the old pCPU. This can
result in the same ASID being used by multiple vCPUs on the old pCPU.

Example scenario:
- vCPU runs on pCPU A, vmcb01 is active, asid=1.
- vCPU migrates to pCPU B, vmcb01 pCPU changes, new asid=2.
- Another vCPU runs on pCPU A and allocates asid=2 as well.
- vCPU migrates back to pCPU A, and then switches to vmcb02 before it
  runs again with vmcb01.
- No pCPU switch is detected for vmcb02, so VMRUN is done with asid=2.
- Two vCPUs end up using asid=2 on the same pCPU.

Keep the VMCB dirtying to the active VMCB only. Clean bits are tracked
by a pCPU for each VMCB, so do not unnecessarily dirty a VMCB if its
pCPU does not change.

Additionally, initialize the tracked pCPU for vmcb02 to -1 on nested
enablement as hardening, so that a new ASID allocation is always
triggered when nested is disabled and re-enabled.

No performance regression was noticed when overcommitting L1 vCPUs in L0
(to force rescheduling), pinning L1 <-> L2 vCPUs, and running CPUID in a
tight loop bouncing between 2 vCPUs in L2.

An alternative (and perhaps more proper) fix would be tracking the ASID
per-VMCB instead (e.g. [1]). However, that's a more involved change, and
it would result in having different ASIDs for L1 and L2 without actually
properly maintaining them. It would probably work because all TLB
flushes target the current VMCB, and the other VMCB is always flushed on
nested transitions, but the code ends up in an arguably more fragile
state. Punt a proper clean fix to an incoming (and overdue) overhaul of
SVM's ASID usage [2].

[1] https://lore.kernel.org/lkml/20250205182402.2147495-2-yosry.ahmed@linux.dev
[2] https://lore.kernel.org/kvm/20260728003557.1136583-1-yosry@kernel.org

Cc: stable@vger.kernel.org
Reported-by: Stefan Teodorescu <fane@google.com>
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Nikunj A Dadhania <nikunj@amd.com>
Link: https://patch.msgid.link/20260903223258.486034-1-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

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