KVM: x86/mmu: Don't update nested guest's paging bitmasks if CR0.PG=0

Don't bother updating the bitmasks and last-leaf information if paging is
disabled as the metadata will never be used.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-43-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>