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/net/hamradio/6pack.c

@@ -34,7 +34,6 @@
 #include <linux/ip.h>
 #include <linux/tcp.h>
 #include <linux/semaphore.h>
-#include <linux/compat.h>
 #include <linux/refcount.h>
 
 #define SIXPACK_VERSION    "Revision: 0.3.0"
@@ -752,23 +751,6 @@ static int sixpack_ioctl(struct tty_struct *tty, struct file *file,
 	return err;
 }
 
-#ifdef CONFIG_COMPAT
-static long sixpack_compat_ioctl(struct tty_struct * tty, struct file * file,
-				unsigned int cmd, unsigned long arg)
-{
-	switch (cmd) {
-	case SIOCGIFNAME:
-	case SIOCGIFENCAP:
-	case SIOCSIFENCAP:
-	case SIOCSIFHWADDR:
-		return sixpack_ioctl(tty, file, cmd,
-				(unsigned long)compat_ptr(arg));
-	}
-
-	return -ENOIOCTLCMD;
-}
-#endif
-
 static struct tty_ldisc_ops sp_ldisc = {
 	.owner		= THIS_MODULE,
 	.magic		= TTY_LDISC_MAGIC,
@@ -776,9 +758,6 @@ static struct tty_ldisc_ops sp_ldisc = {
 	.open		= sixpack_open,
 	.close		= sixpack_close,
 	.ioctl		= sixpack_ioctl,
-#ifdef CONFIG_COMPAT
-	.compat_ioctl	= sixpack_compat_ioctl,
-#endif
 	.receive_buf	= sixpack_receive_buf,
 	.write_wakeup	= sixpack_write_wakeup,
 };