Nove the kvm_unlink_unsync_page() call out of kvm_sync_page() and into
it's sole caller, and fold __kvm_sync_page() into kvm_sync_page() since
the latter becomes a pure pass-through.  There really should be no reason
for code to do a complete sync of a shadow page outside of the full
kvm_mmu_sync_roots(), e.g. the one use case that creeped in turned out to
be flawed and counter-productive.

Drop the stale comment about @sp->gfn needing to be write-protected, as
it directly contradicts the kvm_mmu_get_page() usage.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-13-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>