net: split out SIOCDEVPRIVATE handling from dev_ioctl

SIOCDEVPRIVATE ioctl commands are mainly used in really old
drivers, and they have a number of problems:

- They hide behind the normal .ndo_do_ioctl function that
  is also used for other things in modern drivers, so it's
  hard to spot a driver that actually uses one of these

- Since drivers use a number different calling conventions,
  it is impossible to support compat mode for them in
  a generic way.

- With all drivers using the same 16 commands codes, there
  is no way to introspect the data being passed through
  things like strace.

Add a new net_device_ops callback pointer, to address the
first two of these. Separating them from .ndo_do_ioctl
makes it easy to grep for drivers with a .ndo_siocdevprivate
callback, and the unwieldy name hopefully makes it easier
to spot in code review.

By passing the ifreq structure and the ifr_data pointer
separately, it is no longer necessary to overload these,
and the driver can use either one for a given command.

Cc: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1361,6 +1361,9 @@ struct net_device_ops {
 	int			(*ndo_validate_addr)(struct net_device *dev);
 	int			(*ndo_do_ioctl)(struct net_device *dev,
 					        struct ifreq *ifr, int cmd);
+	int			(*ndo_siocdevprivate)(struct net_device *dev,
+						      struct ifreq *ifr,
+						      void __user *data, int cmd);
 	int			(*ndo_set_config)(struct net_device *dev,
 					          struct ifmap *map);
 	int			(*ndo_change_mtu)(struct net_device *dev,