KVM: MMU: don't atomicly set spte if it's not present

If the old mapping is not present, the spte.a is not lost, so no need
atomic operation to set it

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>