KVM: nVM: Ensure INVVPID is emulated on the correct physical CPU

When emulating INVVPID, KVM executes INVVPID on the physical CPU using
vpid02 (instead of the L1 assigned VPID), after doing some validations
on the operands. However, it is possible that the physical CPU KVM
executes INVVPID on is different from the CPU L2 is running on.

For example, in the following scenario:
- L2 runs on CPU #1 and exits to L1 (vmx->nested.vmcs02.cpu=1)
- L1 migrates to CPU #2 and executes INVVPID
- KVM executes INVVPID on CPU #2
- L1 migrates back to CPU #1 and runs L2 (vmx->nested.vmcs02.cpu=1)

The TLB entries on CPU #1 are never invalidated, because INVVPID was
executed on CPU #2, and vmcs02 never ran on a different pCPU (i.e.
vmx_vcpu_load_vmcs() will *not* request KVM_REQ_TLB_FLUSH).

Ensure that INVVPID is being executed on the same pCPU that L2 last ran
on, and if not, fallback to clearing last_vpid=0 to trigger a full VPID
flush on the next nested VM-Enter (as KVM will detect L1 using a
different VPID for L2). If L2 ends up running on a different pCPU, KVM
will flush the TLB anyway through vmx_vcpu_load_vmcs().

Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20260616214652.2157032-4-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

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