pstore/ram: (really) fix undefined usage of rounddown_pow_of_two

Previous attempt to fix was b042e47491ba5f487601b5141a3f1d8582304170

Suggested use of is_power_of_2() was bogus because is_power_of_2(0) is
false (documented behaviour).

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/ram.c

@@ -421,11 +421,11 @@ static int ramoops_probe(struct platform_device *pdev)
 		goto fail_out;
 	}
 
-	if (!is_power_of_2(pdata->record_size))
+	if (pdata->record_size && !is_power_of_2(pdata->record_size))
 		pdata->record_size = rounddown_pow_of_two(pdata->record_size);
-	if (!is_power_of_2(pdata->console_size))
+	if (pdata->console_size && !is_power_of_2(pdata->console_size))
 		pdata->console_size = rounddown_pow_of_two(pdata->console_size);
-	if (!is_power_of_2(pdata->ftrace_size))
+	if (pdata->ftrace_size && !is_power_of_2(pdata->ftrace_size))
 		pdata->ftrace_size = rounddown_pow_of_two(pdata->ftrace_size);
 
 	cxt->dump_read_cnt = 0;