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>