KVM: x86/mmu: Remove redundant spte present check in mmu_set_spte

Drop an unnecessary is_shadow_present_pte() check when updating the rmaps
after installing a non-MMIO SPTE.  set_spte() is used only to create
shadow-present SPTEs, e.g. MMIO SPTEs are handled early on, mmu_set_spte()
runs with mmu_lock held for write, i.e. the SPTE can't be zapped between
writing the SPTE and updating the rmaps.

Opportunistically combine the "new SPTE" logic for large pages and rmaps.

No functional change intended.

Suggested-by: Ben Gardon <bgardon@google.com>

Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Message-Id: <20210803044607.599629-2-mizhang@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>