openvswitch: Derive IP protocol number for IPv6 later frags

Currently, OVS only parses the IP protocol number for the first
IPv6 fragment, but sets the IP protocol number for the later fragments
to be NEXTHDF_FRAGMENT.  This patch tries to derive the IP protocol
number for the IPV6 later frags so that we can match that.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>