KVM: PPC: fix leakage of error page in kvmppc_patch_dcbz()

Add kvm_release_page_clean() after is_error_page() to avoid
leakage of error page.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>