KVM: Remove unnecessary export of kvm_{inc,dec}_notifier_count()

Don't export KVM's MMU notifier count helpers, under no circumstance
should any downstream module, including x86's vendor code, have a
legitimate reason to piggyback KVM's MMU notifier logic.  E.g in the x86
case, only KVM's MMU should be elevating the notifier count, and that
code is always built into the core kvm.ko module.

Fixes: edb298c6 ("KVM: x86/mmu: bump mmu notifier count in kvm_zap_gfn_range")
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210902175951.1387989-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>