KVM: x86: Bail to userspace if emulation of atomic user access faults

Exit to userspace when emulating an atomic guest access if the CMPXCHG on
the userspace address faults.  Emulating the access as a write and thus
likely treating it as emulated MMIO is wrong, as KVM has already
confirmed there is a valid, writable memslot.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220202004945.2540433-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>