KVM: MMU: move zap/flush to kvm_mmu_get_page

kvm_mmu_get_page is the only caller of kvm_sync_page_transient
and kvm_sync_pages.  Moving the handling of the invalid_list there
removes the need for the underdocumented kvm_sync_page_transient
function.

Reviewed-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>