decnet: fix DN_IFREQ_SIZE

Digging through the ioctls with Al because of the previous
patches, we found that on 64-bit decnet's dn_dev_ioctl()
is wrong, because struct ifreq::ifr_ifru is actually 24
bytes (not 16 as expected from struct sockaddr) due to the
ifru_map and ifru_settings members.

Clearly, decnet expects the ioctl to be called with a struct
like
  struct ifreq_dn {
    char ifr_name[IFNAMSIZ];
    struct sockaddr_dn ifr_addr;
  };

since it does
  struct ifreq *ifr = ...;
  struct sockaddr_dn *sdn = (struct sockaddr_dn *)&ifr->ifr_addr;

This means that DN_IFREQ_SIZE is too big for what it wants on
64-bit, as it is
  sizeof(struct ifreq) - sizeof(struct sockaddr) +
  sizeof(struct sockaddr_dn)

This assumes that sizeof(struct sockaddr) is the size of ifr_ifru
but that isn't true.

Fix this to use offsetof(struct ifreq, ifr_ifru).

This indeed doesn't really matter much - the result is that we
copy in/out 8 bytes more than we should on 64-bit platforms. In
case the "struct ifreq_dn" lands just on the end of a page though
it might lead to faults.

As far as I can tell, it has been like this forever, so it seems
very likely that nobody cares.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>