KVM: x86/mmu: Automatically update iter->old_spte if cmpxchg fails

Consolidate a bunch of code that was manually re-reading the spte if the
cmpxchg failed. There is no extra cost of doing this because we already
have the spte value as a result of the cmpxchg (and in fact this
eliminates re-reading the spte), and none of the call sites depend on
iter->old_spte retaining the stale spte value.

Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220119230739.2234394-4-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>