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>

Documentation/networking/netdevices.rst

@@ -222,6 +222,13 @@ ndo_do_ioctl:
 	Synchronization: rtnl_lock() semaphore.
 	Context: process
 
+ndo_siocdevprivate:
+	Synchronization: rtnl_lock() semaphore.
+	Context: process
+
+	This is used to implement SIOCDEVPRIVATE ioctl helpers.
+	These should not be added to new drivers, so don't use.
+
 ndo_get_stats:
 	Synchronization: rtnl_lock() semaphore, dev_base_lock rwlock, or RCU.
 	Context: atomic (can't sleep under rwlock or RCU)