powerpc/mm: Add a helper to select PAGE_KERNEL_RO or PAGE_READONLY

In a couple of places there is a need to select whether read-only
protection of shadow pages is performed with PAGE_KERNEL_RO or with
PAGE_READONLY.

Add a helper to avoid duplicating the choice.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9f33f44b9cd741c4a02b3dce7b8ef9438fe2cd2a.1566382750.git.christophe.leroy@c-s.fr

arch/powerpc/mm/kasan/kasan_init_32.c

@@ -12,6 +12,14 @@
 #include <asm/code-patching.h>
 #include <mm/mmu_decl.h>
 
+static pgprot_t kasan_prot_ro(void)
+{
+	if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE))
+		return PAGE_READONLY;
+
+	return PAGE_KERNEL_RO;
+}
+
 static void kasan_populate_pte(pte_t *ptep, pgprot_t prot)
 {
 	unsigned long va = (unsigned long)kasan_early_shadow_page;
@@ -26,6 +34,7 @@ static int __ref kasan_init_shadow_page_tables(unsigned long k_start, unsigned l
 {
 	pmd_t *pmd;
 	unsigned long k_cur, k_next;
+	pgprot_t prot = kasan_prot_ro();
 
 	pmd = pmd_offset(pud_offset(pgd_offset_k(k_start), k_start), k_start);
 
@@ -43,10 +52,7 @@ static int __ref kasan_init_shadow_page_tables(unsigned long k_start, unsigned l
 
 		if (!new)
 			return -ENOMEM;
-		if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE))
-			kasan_populate_pte(new, PAGE_READONLY);
-		else
-			kasan_populate_pte(new, PAGE_KERNEL_RO);
+		kasan_populate_pte(new, prot);
 
 		smp_wmb(); /* See comment in __pte_alloc */
 
@@ -103,10 +109,9 @@ static int __ref kasan_init_region(void *start, size_t size)
 
 static void __init kasan_remap_early_shadow_ro(void)
 {
-	if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE))
-		kasan_populate_pte(kasan_early_shadow_pte, PAGE_READONLY);
-	else
-		kasan_populate_pte(kasan_early_shadow_pte, PAGE_KERNEL_RO);
+	pgprot_t prot = kasan_prot_ro();
+
+	kasan_populate_pte(kasan_early_shadow_pte, prot);
 
 	flush_tlb_kernel_range(KASAN_SHADOW_START, KASAN_SHADOW_END);
 }