KVM: x86/mmu: Check mmu->sync_page pointer in kvm_sync_page_check()

Assert that mmu->sync_page is non-NULL as part of the sanity checks
performed before attempting to sync a shadow page.  Explicitly checking
mmu->sync_page is all but guaranteed to be redundant with the existing
sanity check that the MMU is indirect, but the cost is negligible, and
the explicit check also serves as documentation.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Link: https://lore.kernel.org/r/20230216154115.710033-4-jiangshanlai@gmail.com


[sean: increase verbosity of changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>