KVM: x86/mmu: Set mmu->sync_page as NULL for direct paging

mmu->sync_page for direct paging is never called.

And both mmu->sync_page and mm->invlpg only make sense in shadow paging.
Setting mmu->sync_page as NULL for direct paging makes it consistent
with mm->invlpg which is set NULL for the case.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Link: https://lore.kernel.org/r/20230216154115.710033-5-jiangshanlai@gmail.com


Signed-off-by: Sean Christopherson <seanjc@google.com>