datapath: Add ipv6_find_hdr() compatibility function.

datapath: Add ipv6_find_hdr() compatibility function.

Implementation of the IPv6 'set' action depends on ipv6_find_exthdr()
function to find routing header. By looking at the routing headers,
it is possible to tell, whether L4 checksums will need to be
recalculated, whenever the destination address is changed in
the main IPv6 header.

This function will need to be customized so that it would skip all
routing headers, where segements_left is equal to 0.

Signed-off-by: Ansis Atteka <aatteka@nicira.com>