net: Add interface to bridge when SIOCBRADDIF isn't available

The bridge helper uses the SIOCBRADDIF ioctl to add an inteface to
a bridge.  SIOCBRADDIF is not available on old Linux versions.  This
patch adds support to use the SIOCDEVPRIVATE ioctl with BRCTL_ADD_IF
if SIOCBRADDIF is not available.

Reported-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>