Add checks for bad wildcarded flows.

This commit add functions to check if a masked field is valid.
Fix some match errors

- Fix error while matching masked IPv6, IPv4 and ethernet addresses.
- Fix default metadata value. The older value was leading to wrong flow misses.
Implement SET_NW_TTL and DEC_NW_TTL for IPv6 packets

The implementation of SET_NW_TTL and DEC_NW_TTL was not
considering IPv6 packets. This commit fix this, setting and decrementing
the ipv6 hop_limit on respective action.
Fix OXM_OF_IPV6_ND_SLL and OXM_OF_IPV6_ND_TLL set_field action.

This commit fix set_field action of IPv6 Neighbor Discovery TLL and SLL,
correcting the value memcpy position and recalculating the ICMPv6 checksum.
Merge remote-tracking branch 'upstream/master'