Revert "KVM: async_pf: Fix #DF due to inject "Page not Present" and "Page Ready" exceptions simultaneously"

Commit 9a6e7c39 (""KVM: async_pf: Fix #DF due to inject "Page not
Present" and "Page Ready" exceptions simultaneously") added a protection
against 'page ready' notification coming before 'page not present' is
delivered. This situation seems to be impossible since commit 2a266f23
("KVM MMU: check pending exception before injecting APF) which added
'vcpu->arch.exception.pending' check to kvm_can_do_async_pf.

On x86, kvm_arch_async_page_present() has only one call site:
kvm_check_async_pf_completion() loop and we only enter the loop when
kvm_arch_can_inject_async_page_present(vcpu) which when async pf msr
is enabled, translates into kvm_can_do_async_pf().

There is also one problem with the cancellation mechanism. We don't seem
to check that the 'page not present' notification we're canceling matches
the 'page ready' notification so in theory, we may erroneously drop two
valid events.

Revert the commit.
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200525144125.143875-2-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>