CVE-2026-46071: KVM: nSVM: Avoid clearing VMCB_LBR in vmcb12

In the Linux kernel, the following vulnerability has been resolved:

KVM: nSVM: Avoid clearing VMCBLBR in vmcb12

svmcopylbrs() always marks VMCBLBR dirty in the destination VMCB. However, nestedsvmvmexit() uses it to copy LBRs to vmcb12, and clearing clean bits in vmcb12 is not architecturally defined.

Move vmcbmarkdirty() to callers and drop it for vmcb12.

This also facilitates incoming refactoring that does not pass the entire VMCB to svmcopylbrs().