conntrack: add generic IP protocol support

conntrack: add generic IP protocol support

Currently, userspace conntrack only tracks TCP, UDP, and ICMP, and all
other IP protocols are discarded, and the +inv state is returned. This
is not in line with the kernel conntrack. Where if no L4 information can
be extracted it's treated as generic L3. The change below mimics the
behavior of the kernel.

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>