KVM: Don't follow an atomic operation by a non-atomic one

Currently emulated atomic operations are immediately followed by a non-atomic
operation, so that kvm_mmu_pte_write() can be invoked.  This updates the mmu
but undoes the whole point of doing things atomically.

Fix by only performing the atomic operation and the mmu update, and avoiding
the non-atomic write.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>