KVM: MMU: Use list_for_each_entry_safe in kvm_mmu_commit_zap_page()

We are traversing the linked list, invalid_list, deleting each entry by
kvm_mmu_free_page().  _safe version is there for such a case.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>