note/noteram: add FIONREAD ioctl to report unread buffer size

Add handling for the FIONREAD ioctl in noteram_ioctl to return the
number of unread bytes in the circular note buffer (noteram_unread_length()).
Validate the argument pointer and return -EINVAL if it is NULL.

Signed-off-by: yukangzhi <yukangzhi@xiaomi.com>

drivers/note/noteram_driver.c

@@ -604,6 +604,18 @@ static int noteram_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
           }
         break;
 
+      case FIONREAD:
+        if (arg == 0)
+          {
+            ret = -EINVAL;
+          }
+        else
+          {
+            *(FAR unsigned int *)arg = noteram_unread_length(drv);
+            ret = OK;
+          }
+        break;
+
       default:
           break;
     }