[NET]: Convert link modification to new netlink api

Transforms do_setlink() into rtnl_setlink() using the new
netlink api. A warning message printed to the console is
added in the event that a change request fails while part
of the change request has been comitted already. The ioctl()
based nature of net devices makes it almost impossible to
move on to atomic netlink operations without obsoleting
some of the functionality.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>