KVM: PPC: Book3S: Mark expected switch fall-through

Fix the error below triggered by `-Wimplicit-fallthrough`, by tagging
it as an expected fall-through.

    arch/powerpc/kvm/book3s_32_mmu.c: In function ‘kvmppc_mmu_book3s_32_xlate_pte’:
    arch/powerpc/kvm/book3s_32_mmu.c:241:21: error: this statement may fall through [-Werror=implicit-fallthrough=]
          pte->may_write = true;
          ~~~~~~~~~~~~~~~^~~~~~
    arch/powerpc/kvm/book3s_32_mmu.c:242:5: note: here
         case 3:
         ^~~~
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_32_mmu.c

@@ -239,6 +239,7 @@ static int kvmppc_mmu_book3s_32_xlate_pte(struct kvm_vcpu *vcpu, gva_t eaddr,
 				case 2:
 				case 6:
 					pte->may_write = true;
+					/* fall through */
 				case 3:
 				case 5:
 				case 7: