lib/flow: fix minimatch_extract() ICMPv6 parsing

This patch addresses two bugs related to ICMPv6(NDP) packets:

- In miniflow_extract() push the words in the correct order
- In parse_icmpv6() use sizeof struct, not size of struct *

match_wc_init() has been modified, to include the nd_target field
when the transport layer protocol is ICMPv6

A testcase has been added to detect the bugs.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>