KVM: MMU: stop dereferencing vcpu->arch.mmu to get the context for MMU init

kvm_init_shadow_mmu() was actually the only function that could be called
with different vcpu->arch.mmu values.  Now that kvm_init_shadow_npt_mmu()
is separated from kvm_init_shadow_mmu(), we always know the MMU context
we need to use and there is no need to dereference vcpu->arch.mmu pointer.

Based on a patch by Vitaly Kuznetsov <vkuznets@redhat.com>.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200710141157.1640173-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>