compat: add struct compat_ifreq etc to compat.h

In order to move socket ioctl conversion code into multiple
places in the socket code, we need a common defintion of
the data structures it uses.

Also change the name from ifreq32 to compat_ifreq to
follow the naming convention for compat.h

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