ethdev: add IPv6 fragment extension header item

ethdev: add IPv6 fragment extension header item

Applications handling fragmented IPv6 packets need to match on IPv6
fragment extension header, in order to identify the fragments order
and location in the packet.
This patch introduces the IPv6 fragment extension header item,
proposed in [1].

Relevant definitions are moved from lib/librte_ip_frag/rte_ip_frag.h
to lib/librte_net/rte_ip.h, as they are needed for IPv6 header handling.
struct ipv6_extension_fragment renamed to rte_ipv6_fragment_ext to
adapt it to the common naming convention.

Default mask is not defined, since all fields are optional.

[1] http://mails.dpdk.org/archives/dev/2020-March/160255.html

Signed-off-by: Dekel Peled <dekelp@nvidia.com>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>