datapath: Fixup RTNL ops for kernel 4.13

datapath: Fixup RTNL ops for kernel 4.13

The RTNL ops validate and newlink functions now take the extended
netlink ack parameter.  Use the new HAVE_EXT_ACK_IN_RTNL_LINKOPS
define to check if the additional parameter is present and add the
parameter if so.

While in the modules remove the checks for Linux kernels < 2.3.39
since they are no longer supported since 2.5.x.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Andy Zhou <azhou@ovn.org>