ipv6: add struct hop_jumbo_hdr definition

Following patches will need to add and remove local IPv6 jumbogram
options to enable BIG TCP.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>