powerpc/mmu: add is_strict_kernel_rwx() helper

Add a helper to know whether STRICT_KERNEL_RWX is enabled.

This is based on rodata_enabled flag which is defined only
when CONFIG_STRICT_KERNEL_RWX is selected.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/init_32.c

@@ -108,12 +108,10 @@ static void __init MMU_setup(void)
 		__map_without_bats = 1;
 		__map_without_ltlbs = 1;
 	}
-#ifdef CONFIG_STRICT_KERNEL_RWX
-	if (rodata_enabled) {
+	if (strict_kernel_rwx_enabled()) {
 		__map_without_bats = 1;
 		__map_without_ltlbs = 1;
 	}
-#endif
 }
 
 /*