KVM: x86/mmu: Invert @can_unsync and renamed to @synchronizing

Invert the polarity of "can_unsync" and rename the parameter to
"synchronizing" to allow a future change to set the Accessed bit if KVM
is synchronizing an existing SPTE.  Querying "can_unsync" in that case is
nonsensical, as the fact that KVM can't unsync SPTEs doesn't provide any
justification for setting the Accessed bit.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-7-seanjc@google.com>