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

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 <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Reviewed-by: Sean Christopherson <[email protected]>
Signed-off-by: David Matlack <[email protected]>
Message-Id: <20220119230739.2234394[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>