KVM: x86/mmu: Remove lpage_is_disallowed() check from set_spte()

Remove the late "lpage is disallowed" check from set_spte() now that the
initial check is performed after acquiring mmu_lock.  Fold the guts of
the remaining helper, __mmu_gfn_lpage_is_disallowed(), into
kvm_mmu_hugepage_adjust() to eliminate the unnecessary slot !NULL check.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>