powerpc/ptdump: Fix W+X verification

Verification cannot rely on simple bit checking because on some
platforms PAGE_RW is 0, checking that a page is not W means
checking that PAGE_RO is set instead of checking that PAGE_RW
is not set.

Use pte helpers instead of checking bits.

Fixes: 453d87f6 ("powerpc/mm: Warn if W+X pages found on boot")
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0d894839fdbb19070f0e1e4140363be4f2bb62fc.1578989540.git.christophe.leroy@c-s.fr

arch/powerpc/mm/ptdump/ptdump.c

@@ -175,10 +175,12 @@ static void dump_addr(struct pg_state *st, unsigned long addr)
 
 static void note_prot_wx(struct pg_state *st, unsigned long addr)
 {
+	pte_t pte = __pte(st->current_flags);
+
 	if (!IS_ENABLED(CONFIG_PPC_DEBUG_WX) || !st->check_wx)
 		return;
 
-	if (!((st->current_flags & pgprot_val(PAGE_KERNEL_X)) == pgprot_val(PAGE_KERNEL_X)))
+	if (!pte_write(pte) || !pte_exec(pte))
 		return;
 
 	WARN_ONCE(1, "powerpc/mm: Found insecure W+X mapping at address %p/%pS\n",