powerpc/mm: Add MMU_FTR_KERNEL_RO to possible feature mask

Without this we will always find the feature disabled.

Fixes: 984d7a1e ("powerpc/mm: Fixup kernel read only mapping")
Cc: stable@vger.kernel.org # v4.7+
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/mmu.h

@@ -136,6 +136,7 @@ enum {
 		MMU_FTR_NO_SLBIE_B | MMU_FTR_16M_PAGE | MMU_FTR_TLBIEL |
 		MMU_FTR_LOCKLESS_TLBIE | MMU_FTR_CI_LARGE_PAGE |
 		MMU_FTR_1T_SEGMENT | MMU_FTR_TLBIE_CROP_VA |
+		MMU_FTR_KERNEL_RO |
 #ifdef CONFIG_PPC_RADIX_MMU
 		MMU_FTR_TYPE_RADIX |
 #endif