KVM: SVM: Mark VMCB dirty before processing incoming snp_vmsa_gpa

KVM: SVM: Mark VMCB dirty before processing incoming snp_vmsa_gpa

Mark the VMCB dirty, i.e. zero control.clean, prior to handling the new
VMSA.  Nothing in the VALID_PAGE() case touches control.clean, and
isolating the VALID_PAGE() code will allow simplifying the overall logic.

Note, the VMCB probably doesn't need to be marked dirty when the VMSA is
invalid, as KVM will disallow running the vCPU in such a state.  But it
also doesn't hurt anything.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20250227012541.3234589-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>