KVM: MMU: use kvm_release_pfn_clean to release pfn

The current code depends on the fact that fault_page is the normal page,
however, we will use the error code instead of these dummy pages in the
later patch, so we use kvm_release_pfn_clean to release pfn which will
release the error code properly

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>