datapath: supply a dummy err_handler of gre_cisco_protocol to prevent kernel crash

When use gre vport, openvswitch register a gre_cisco_protocol but
does not supply a err_handler with it. The gre_cisco_err() in
net/ipv4/gre_demux.c expect err_handler be provided with the
gre_cisco_protocol implementation, and call ->err_handler() without
existence check, cause the kernel crash.

This patch provide a err_handler to fix this bug.

Signed-off-by: Wei Zhang <asuka.com@163.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>