net: copy socket ioctl code to net/socket.h

This makes an identical copy of the socket compat_ioctl code
from fs/compat_ioctl.c to net/socket.c, as a preparation
for moving the functionality in a way that can be easily
reviewed.

The code is hidden inside of #if 0 and gets activated in the
patch that will make it work.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>