datapath: Do not fail to load on gre protocol conflict

datapath: Do not fail to load on gre protocol conflict

The ERSPAN feature depends on the gre kernel module so on systems where
the ERSPAN feature isn't supported the openvswitch kernel module would
attempt to grab the ipv4 GRE protocol entry point and would fail to load
if it could not.

This patch modifies openvswitch to not fail to load when the gre kernel
module is loaded and instead it will print a warning message to the
kernel system log indicating that the ERSPAN feature may not be
available.

We need this patch because users are experiencing failures due to the
conflicts and high priority bugs are resulting.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>