KVM: MMU: remove call to kvm_mmu_pte_write from walk_addr

There is no reason to update the shadow pte here because the guest pte
is only changed to dirty state.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>