KVM/MMU: Fix comment in walk_shadow_page_lockless_end()

kvm_commit_zap_page() has been renamed to kvm_mmu_commit_zap_page()
This patch is to fix the commit.

Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>