datapath: Fix checksum calculation when modifying ICMPv6 packets.

The checksum of ICMPv6 packets uses the IP pseudoheader as part of
the calculation, unlike ICMP in IPv4. This was not implemented,
which means that modifying the IP addresses of an ICMPv6 packet
would cause the checksum to no longer be correct as the psuedoheader
did not match.

Reported-by: Neal Shrader <icosahedral@gmail.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>