datapath: Fix builds on older kernels.

datapath: Fix builds on older kernels.

On older kernels, for example 3.19, the function rt6_get_cookie() is
not available and used with ipv6 config enabled;  it was introduced in
4.2.  Put back the replacement function if it does not exist.
Add a 3.19 version to travis.

CC: Yifeng Sun <pkusunyifeng@gmail.com>
Fixes: bf61b8b1c1db ("datapath: Add support for kernel 4.16.x & 4.17.x.")
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Yifeng Sun <pkusunyifeng@gmail.com>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>