KVM: nSVM: Avoid clearing VMCB_LBR in vmcb12

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().

Fixes: d20c796ca370 ("KVM: x86: nSVM: implement nested LBR virtualization")
Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260303003421.2185681-2-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>