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_gsm.c

@@ -2614,14 +2614,6 @@ static int gsmld_ioctl(struct tty_struct *tty, struct file *file,
 	}
 }
 
-#ifdef CONFIG_COMPAT
-static long gsmld_compat_ioctl(struct tty_struct *tty, struct file *file,
-		       unsigned int cmd, unsigned long arg)
-{
-	return gsmld_ioctl(tty, file, cmd, arg);
-}
-#endif
-
 /*
  *	Network interface
  *
@@ -2833,9 +2825,6 @@ static struct tty_ldisc_ops tty_ldisc_packet = {
 	.flush_buffer    = gsmld_flush_buffer,
 	.read            = gsmld_read,
 	.write           = gsmld_write,
-#ifdef CONFIG_COMPAT
-	.compat_ioctl    = gsmld_compat_ioctl,
-#endif
 	.ioctl           = gsmld_ioctl,
 	.poll            = gsmld_poll,
 	.receive_buf     = gsmld_receive_buf,