powerpc/mm: Fix build break with STRICT_MM_TYPECHECKS && DEBUG_PAGEALLOC

If both STRICT_MM_TYPECHECKS and DEBUG_PAGEALLOC are enabled, the code
in kernel_map_linear_page() is built, and so we fail with:

  arch/powerpc/mm/hash_utils_64.c:1478:2:
  error: incompatible type for argument 1 of 'htab_convert_pte_flags'

Fix it by using pgprot_val().
Reported-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/hash_utils_64.c

@@ -1475,7 +1475,7 @@ static void kernel_map_linear_page(unsigned long vaddr, unsigned long lmi)
 	unsigned long hash;
 	unsigned long vsid = get_kernel_vsid(vaddr, mmu_kernel_ssize);
 	unsigned long vpn = hpt_vpn(vaddr, vsid, mmu_kernel_ssize);
-	unsigned long mode = htab_convert_pte_flags(PAGE_KERNEL);
+	unsigned long mode = htab_convert_pte_flags(pgprot_val(PAGE_KERNEL));
 	long ret;
 
 	hash = hpt_hash(vpn, PAGE_SHIFT, mmu_kernel_ssize);