KVM MMU: check pending exception before injecting APF

KVM MMU: check pending exception before injecting APF

commit 2a266f23550be997d783f27e704b9b40c4010292 upstream.

For example, when two APF's for page ready happen after one exit and
the first one becomes pending, the second one will result in #DF.
Instead, just handle the second page fault synchronously.

Reported-by: Ross Zwisler <zwisler@gmail.com>
Message-ID: <CAOxpaSUBf8QoOZQ1p4KfUp0jq76OKfGY4Uxs-Gg8ngReD99xww@mail.gmail.com>
Reported-by: Alec Blayne <ab@tevsa.net>
Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>