gre: Resolve gre receive issues

gre: Resolve gre receive issues

On newer Linux kernels or on older kernels such as Red Hat that backport
from newer upstream Linux kernel releases the built-in gre kernel module
will interfere with OVS gre code in the receive path.  Fix this up by
placing the gre kernel code within the openvswitch driver so it will
not have to depend on the built-in gre kernel module.

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