KVM: fix direction of dependency on MMU notifiers

KVM_GENERIC_MEMORY_ATTRIBUTES requires the generic MMU notifier code, because
it uses kvm_mmu_invalidate_begin/end.  However, it would not work with a bespoke
implementation of MMU notifiers that does not use KVM_GENERIC_MMU_NOTIFIER,
because most likely it would not synchronize correctly on invalidation.  So
the right thing to do is to note the problematic configuration if the
architecture does not select itself KVM_GENERIC_MMU_NOTIFIER; not to
enable it blindly.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>