change semantics of ldisc ->compat_ioctl()

First of all, make it return int.  Returning long when native method
had never allowed that is ridiculous and inconvenient.

More importantly, change the caller; if ldisc ->compat_ioctl() is NULL
or returns -ENOIOCTLCMD, tty_compat_ioctl() will try to feed cmd and
compat_ptr(arg) to ldisc's native ->ioctl().

That simplifies ->compat_ioctl() instances quite a bit - they only
need to deal with ioctls that are neither generic tty ones (those
would get shunted off to tty_ioctl()) nor simple compat pointer ones.

Note that something like TCFLSH won't reach ->compat_ioctl(),
even if ldisc ->ioctl() does handle it - it will be recognized
earlier and passed to tty_ioctl() (and ultimately - ldisc ->ioctl()).

For many ldiscs it means that NULL ->compat_ioctl() does the
right thing.  Those where it won't serve (see e.g. n_r3964.c) are
also easily dealt with - we need to handle the numeric-argument
ioctls (calling the native instance) and, if such would exist,
the ioctls that need layout conversion, etc.

All in-tree ldiscs dealt with.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/tty/n_r3964.c

@@ -134,6 +134,10 @@ static ssize_t r3964_write(struct tty_struct *tty, struct file *file,
 		const unsigned char *buf, size_t nr);
 static int r3964_ioctl(struct tty_struct *tty, struct file *file,
 		unsigned int cmd, unsigned long arg);
+#ifdef CONFIG_COMPAT
+static int r3964_compat_ioctl(struct tty_struct *tty, struct file *file,
+		unsigned int cmd, unsigned long arg);
+#endif
 static void r3964_set_termios(struct tty_struct *tty, struct ktermios *old);
 static __poll_t r3964_poll(struct tty_struct *tty, struct file *file,
 		struct poll_table_struct *wait);
@@ -149,6 +153,9 @@ static struct tty_ldisc_ops tty_ldisc_N_R3964 = {
 	.read = r3964_read,
 	.write = r3964_write,
 	.ioctl = r3964_ioctl,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl = r3964_compat_ioctl,
+#endif
 	.set_termios = r3964_set_termios,
 	.poll = r3964_poll,
 	.receive_buf = r3964_receive_buf,
@@ -1210,6 +1217,21 @@ static int r3964_ioctl(struct tty_struct *tty, struct file *file,
 	}
 }
 
+#ifdef CONFIG_COMPAT
+static int r3964_compat_ioctl(struct tty_struct *tty, struct file *file,
+		unsigned int cmd, unsigned long arg)
+{
+	switch (cmd) {
+	case R3964_ENABLE_SIGNALS:
+	case R3964_SETPRIORITY:
+	case R3964_USE_BCC:
+		return r3964_ioctl(tty, file, cmd, arg);
+	default:
+		return -ENOIOCTLCMD;
+	}
+}
+#endif
+
 static void r3964_set_termios(struct tty_struct *tty, struct ktermios *old)
 {
 	TRACE_L("set_termios");