KVM: MMU: remove the redundant get_written_sptes

get_written_sptes is called twice in kvm_mmu_pte_write, one of them can be
removed

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