socket: fix struct ifreq size in compat ioctl

As reported by Reobert O'Callahan, since Viro's commit to kill
dev_ifsioc() we attempt to copy too much data in compat mode,
which may lead to EFAULT when the 32-bit version of struct ifreq
sits at/near the end of a page boundary, and the next page isn't
mapped.

Fix this by passing the approprate compat/non-compat size to copy
and using that, as before the dev_ifsioc() removal. This works
because only the embedded "struct ifmap" has different size, and
this is only used in SIOCGIFMAP/SIOCSIFMAP which has a different
handler. All other parts of the union are naturally compatible.

This fixes https://bugzilla.kernel.org/show_bug.cgi?id=199469.

Fixes: bf440573

 ("kill dev_ifsioc()")
Reported-by: Robert O'Callahan <robert@ocallahan.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>