kvm: arm/arm64: Remove spurious WARN_ON

On a 4-level page table pgd entry can be empty, unlike a 3-level
page table. Remove the spurious WARN_ON() in stage_get_pud().
Acked-by: NChristoffer Dall <cdall@kernel.org>
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Reviewed-by: NEric Auger <eric.auger@redhat.com>
Signed-off-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>

virt/kvm/arm/mmu.c

@@ -1005,7 +1005,7 @@ static pud_t *stage2_get_pud(struct kvm *kvm, struct kvm_mmu_memory_cache *cache
 	pud_t *pud;
 
 	pgd = kvm->arch.pgd + stage2_pgd_index(addr);
-	if (WARN_ON(stage2_pgd_none(*pgd))) {
+	if (stage2_pgd_none(*pgd)) {
 		if (!cache)
 			return NULL;
 		pud = mmu_memory_cache_alloc(cache);