slip/plip: use ndo_siocdevprivate

slip and plip both use a couple of SIOCDEVPRIVATE ioctl
commands that overload the ifreq layout in a way that is
incompatible with compat mode.

Convert to use ndo_siocdevprivate to allow passing the
data this way, but return an error in compat mode anyway
because the private structure is still incompatible.

This could be fixed as well to make compat work properly.

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