KVM: arm64: Fix build error in user_mem_abort()

The PUD and PMD are folded into PGD when the following options are
enabled. In that case, PUD_SHIFT is equal to PMD_SHIFT and we fail
to build with the indicated errors:

   CONFIG_ARM64_VA_BITS_42=y
   CONFIG_ARM64_PAGE_SHIFT=16
   CONFIG_PGTABLE_LEVELS=3

   arch/arm64/kvm/mmu.c: In function ‘user_mem_abort’:
   arch/arm64/kvm/mmu.c:798:2: error: duplicate case value
     case PMD_SHIFT:
     ^~~~
   arch/arm64/kvm/mmu.c:791:2: note: previously used here
     case PUD_SHIFT:
     ^~~~

This fixes the issue by skipping the check on PUD huge page when PUD
and PMD are folded into PGD.

Fixes: 2f40c46021 ("KVM: arm64: Use fallback mapping sizes for contiguous huge page sizes")
Reported-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201103003009.32955-1-gshan@redhat.com

arch/arm64/kvm/mmu.c

@@ -788,10 +788,12 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	}
 
 	switch (vma_shift) {
+#ifndef __PAGETABLE_PMD_FOLDED
 	case PUD_SHIFT:
 		if (fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
 			break;
 		fallthrough;
+#endif
 	case CONT_PMD_SHIFT:
 		vma_shift = PMD_SHIFT;
 		fallthrough;