KVM: x86/mmu: Rename 'mask' to 'spte' in MMIO SPTE helpers

The value returned by make_mmio_spte() is a SPTE, it is not a mask.
Name it accordingly.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>