porting: Add fixes to support kernel 4.15.x

porting: Add fixes to support kernel 4.15.x

This patch enables OVS kernel module to run on kernel 4.15.x.
Two conntrack-related tests failed:
 - conntrack - multiple zones, local
 - conntrack - multi-stage pipeline, local
This might be due to conntrack policy changes for packets coming
from local ports on kernel 4.15. More survey will be done later.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Co-authored-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Gregory Rose <gvrose8192@gmail.com>
Reviewed-by: Gregory Rose <gvrose8192@gmail.com>