KVM: nSVM: Avoid clearing VMCB_LBR in vmcb12

In the Linux kernel, the following vulnerability has been resolved: KVM: nSVM: Avoid clearing VMCB_LBR in vmcb12 svm_copy_lbrs() always marks VMCB_LBR dirty in the destination VMCB. However, nested_svm_vmexit() uses it to copy LBRs to vmcb12, and clearing clean bits in vmcb12 is not architecturally defined. Move vmcb_mark_dirty() to callers and drop it for vmcb12. This also facilitates incoming refactoring that does not pass the entire VMCB to svm_copy_lbrs().