datapath: Use upstream ipv6_find_hdr().

ipv6_find_hdr() already fixed in newer upstram kernel by Ansis, we
can start using this API safely.
This patch also backports fix (ipv6: ipv6_find_hdr restore prev
functionality) to compat ipv6_find_hdr().

CC: Ansis Atteka <aatteka@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>